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CHAPTER 3 Trends in Low Birthweight This chapter analyzes changes in the rate of low birthweight and in the composition of the low birthweight population during the past 10 to 15 years. The proportion of low birthweight infants has declined, but only modestly--from 7.6 percent in 1971 to 6.8 percent in 1981. The decline has not been substantial, despite several favorable develop- ments: the proportion of pregnant women starting prenatal care in the first trimester increased; educational attainment among women rose markedly; the proportions of pregnancies among teenagers and older mothers decreased; and the prevalence of cigarette smoking among women 20 to 44 years of age declined. Not all changes, however, were favorable. For example, the proportion of unmarried pregnant women increased during the same period. The relatively small decline in the low birthweight rate contri- buted in only a minor way to the substantial decline in the overall infant mortality rate. Far more important were reductions in the mortality rates of low birthweight infants. Still, the low birth- weight infant remains at very high risk of mortality. This chapter attempts to understand the recent past in seeking means to reduce future low birthweight rates. Three categories of questions are examined. 1. Have larger reductions in the low birthweight rate begun to appear in recent years? Has a decline in the low birthweight rate been masked by a concomitant reduction in fetal mortality? 2. Is the composition of the low birthweight population changing? In particular, has the decline in low birthweight occurred among very low birthweight infants (1,500 grams or less), who are especially vulnerable to serious morbidity and mortality? Or has the decline occurred among moderately low birthweight infants (1,501-2,500 grams)? Are the changes concentrated in term deliveries (37 or more weeks gestation) or in preterm infants? 3. Has the differential in low birthweight rates between high-risk and low-risk women widened or narrowed? How large is the differential presently? What might be the effect on low birthweight rates of reducing or eliminating certain sociodemographic r isk factors? What might be the impact of improvements in patterns of prenatal care? 94

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95 To answer these questions, the committee analyzed data for the United States as a whole and for five states--California, Massachusetts, Michigan, North Carolina, and Oregon. Although analysis of data from all reporting states gives the broadest picture of trends in low birthwe~ght, examination of the detailed experiences of selected states permits comparison of trends across different populations and jurisdictions. Accordingly, the states were chosen to reflect a wide variation in geography and demographic characteristics. The states also were selected because the vital records data collected in these states are believed to be of high quality, thus permitting additional consideration of U.S. data as a whole, which necessarily include data of widely varying quality. The analysis of national trends and r isk factors was based on published data and unpublished analyses performed by the National Center for Health Statistics. For California, Michigan, and Oregon, the analyses were derived from cross-tabulatzons performed by organi- zations within the respective states. For Massachusetts and North Carolina, the cross-tabulations were performed by the committee from public use tapes. Birth records for the United States as a whole were analyzed for the period 1971-1981 and for individual, selected states by single year, as available, from 1968 to 1982. The salient results of the analyses are reported in Tables 3.1 through 3.13 in this chapter. More detailed tabulations are provided in Tables B.1 through B.13 in Appendix B. The committee recognizes that entries on birth certificates, the original source of both the national and state data presented in this chapter, vary in their accuracy and completeness. Inferences based on reported gestat~onal age, in particular, need to be qualified. Further, while birth records encode the month when prenatal care began and the tote' number of prenatal visits, the content and quality of care cannot be determined from such data alone. Nevertheless, the committee believes that important insights about trends In low birthweight rates and changes in the composition of the low birthweight population can be gained from information available on the birth record. Overall Trends in Low B~rthweight Rates Table 3.1 shows changes in low birthweight rates in the United States and the five selected states from 1971 to 1981. Data by individual year for the United States are given in Appendix Table B.1 and for selected states in Appendix Table B.2. For some states, data are provided for the years 1968 to 1970 also. Overall, from 1971 to 1981, the low birthweight rate for the united States declined by 11 percent. Although low birthweight rates declined by comparable magnitudes in the individual states studied, the decline was more marked for Massachusetts and less significant for North Carolina and Michigan. There is no clear indication, either in Table 3.1 or in the more detailed Tables B.1 and B.2, that low birthweight rates were declining more rapidly in the earlier or later portions of

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96 TABLE 3.1 Low-Weight Births (2,500 Grams or Less) per 1,000 Live Births in the United States and Five States, 1971, 1976, and 1981 ~ 971 1976 1981 Percentage Decrease, 1971-1981a United States 76 72 68 11 California 66 62 58 12 Massachusetts 71 66 59 17 Michigan 77 75 69 10 North Carolina 87 83 79 9 Oregon 57 54 50 12 NOTE: All data include both singleton and multiple births. For 1971 and 1976, low birthweight includes live births weighing 2,500 grams or less. Since 1979, the National Center for Health Statistics has defined low birthweight as less than 2 ,500 grams. Data presented in this table are der ived from ache sources decor ibed in reference no. 1. aComputed from rates calculated to three significant f igures . the 1968-1982 period. (From the data in Table B.2 alone, it is not possible to determine if the apparent stagnation of low l~irthweight rates in Michigan and North Carolina from 1980 to 1982 is of longer- term s ignif icance. J Table 3.2 and Tables B.3 and B.4 show trends in live births and late fetal deaths . I f changes in medical pract' ce or bir th cer tif i- cation have moored increasing numbers of pregnancies from the fetal death category to the live birth category, then the computed trend in low birthweight rates among live births may understate the actual rate of decline in low-weight rates among all pregnancies. Because there is likely to be substantial undercounting of early fetal deaths, and because shifts in classification from fetal deaths to live births are much more 1 i kely in the late fetal death groups, only fetal deaths of 28 or more weeks gestation are included. The data in Table 3.2 suggest that the assessment of trends in low-weight births among live births alone may understate slightly the rate of decline in low birthweight rates. AS Appendix Table B.4 shows, this conclusion is not altered when all fetal deaths of 20 or more weeks gestation are included. Hereafter, only data for live births are considered. The Compos action of Low-Weight Bir ths Table 3.3 depicts changes in very low birthweight (VT~BW) and moderately low birthweight (MLBW) rates in the united States and five states. The data uniformly show that the decline in low birthweight

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97 TABLE 3.2 Low-Weight Births and Late Fetal Deaths per 1,000 Live Births in Four States, 1971, 1976, and 1981 1971 1976 1981 Percentage Decline, 1971-198 1 California 71 66 61 14 Michigan 81a 77 72 11 North Carolina 93 87 82 12 Oregon 60 57 51 15 NOTE: For California, late fetal deaths are defined as fetal deaths weighing 500 grams or more. For other states, late fetal deaths include all fetal deaths of at least 28 weeks gestation. Data presented In this table are derived from the sources described in reference no. 1. Data aData are for 1972, the earliest year available. completely confined to the 1,501-2,500-gram group. With the of Massachusetts, there may have been a slight increase in rates was exception the VLBW rate from 1971 to 1981. Table 3.4 shows more detailed trends in low birthweight rates In relation to gestational age for the selected states. Interpretation of the results needs to be tempered by the uncertain quality of data on duration of pregnancy. Most states did not perform extensive editing to remove or revise implausible gestational age entries. Moreover, birth records with missing data on gestational age were excluded from the analysis. Because data on gestational age reported before 1975 were considered to be less reliable, only the experience from 1975 to 1981 is reported. The results show some var iatic~n from state to state. For Massachusetts and Oregon, there was a slight decline in the rate of preterm births (those of less than 37 weeks durations , while there was no discernable decline in the preterm rate In North Carolina or Michigan. Concomitantly, Massachusetts and Oregon showed declines in preterm low-weight births, while Michigan and North Carolina did not. All states, however, showed a decline in the term low birthweight group. The larger absolute declines In low birthweight rates in Massachusetts and Oregon appear to be attributable to declines in preterm low-weight births. Because the decline in low birthweight appears to be confined to the moderately low birthweight group (Table 3.3), the remaining two rows of Table 3.4 analyze trends in preterm and term moderately low-weight births. The results suggest that the decline in the proportion of 1,501-2,500-gram births reflects mostly a decrease in term infants. These results are consistent with those reported nationally. For the United States during the period 1970 to 198O, Ressel et al.2

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98 TABLE 3.3 Very Low Birthweight (VT~BW) and Moderately Low Birthweight (=BW) Rates per 1,000 Live Births in the United States and Five States, 1971, 1976, and 1981 Weight Categorya 197 1 1976 1981 IJnited States VLBW 11 12 12 MLBW 65 61 56 California VLBW 9 9 10 MLBW 57 52 49 Massachusetts VLBW 10 9 9 MLBW 61 57 SO Mich igan VLBW 12 13 13 MLBW 64 62 57 Nor th Carol ina VLBW 14 14 15 MLBW 74 69 65 Oregon VLBW 8 8 9 MLBW 49 46 41 NATE: Data presented in this table are derived from the sources descr ibed in reference no. 1. every low birthweight defined as 1,500 grams or less; moderately low birthweight as 1,501 to 2 ,500 grams. reported that the preterm low birthweight rate declined from 38 per 1,000 to 36 per 1,000. The term low birthweight rate declined from 36 per 1,000 to 28 per 1,000. Though declines in term ~ ow birthweight were predominant, the state data presented here suggest that those areas with more marked declines in low birthweight rates may be experiencing some additional decline in preterm births. Analysis _ based on a larger number of states is needed to assess this relationship further. Sociodemographic Characteristics Table 3.5 depicts the relationship between low birthweight rates and race in the United States and five states from 1971 to 1981. More detailed data are given in Tables B.1, B.5, and B.6 of Appendix B. For the United States as a whole, the relative decline in white low birthweight rates exceeded the corresponding relative decline in black low birthweight rates. Thus, white low birthweight rates declined by

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99 TABLE 3.4 Preterm, Low-Weight Preterm, and Low-Weight Term Births per 1,000 Live Births of Known Gestational Age in Four States, 1975 and 1981 Massachusetts Michigan North Carolina Oregon 1975 1981 1975 1981 1975 1981 1975 1981 . 71 67 95 94 92 93 66 61 Preterm LBW 33 30 Term LBW 34 29 42 42 42 42 29 25 33 28 38 32 27 22 Preterm MLBW 24 21 31 29 21 19 Term MLBW 33 28 38 31 25 22 NOTE: Data presented in this table are derived from the sources described in reference no. 1. 14 percent from 1971 to 1981, while black rates declined by only 6 percent. The absolute declines among whites and blacks, however, were more comparable. Although quantitative assessment of changes In the white-black differential depends on the measure used, there is no clear closing of the gap. The numbers of black births in Massachusetts and Oregon were rela- t~vely small. Hence, the reported low birthweight rates In those states are subject to greater variability. For the remaining states studied, however, the trends accord with that observed nationally. It is noteworthy that both white and black low birthweight rates vary across states. Michigan's relatively high overall low birthweight rate, for example, appears attributable to the very high low birth- weight rate among blacks In that state. North Carolina's relatively high overa1 1 low birthweight rate, by contrast, reflects elevated rates for both races. The trend in low birthweight among Spanish surnamed whites in California is comparable to that for other whites in the state. For the United States as a whole from 1971 to 1981, Appendix Table B.7 depicts trends In low-weight births according to age of the mother for whites and blacks separately. To display more clearly the relative positions of the various age groups, Table 3.6 computes the relative risks of low birthweight. The relative risk is the ratio of the low birthweight rate for a particular category to the low birthweight rate for all births of the same race in the same year.* Thus, the low birthwe~ght rate among all white births in 1971 was 65.5 per 1,000 (see Appendix Table B.71. Among white mothers under 15 years of age, the rate was 127.8 per 1,000, that is, 1.95 times the overall white rate in 1971 (see Table 3.6~. For selected states, more detailed, absolute rates are given in Appendix Table B.8. AS Table 3.6 shows, the highest *This definition of relative risk is specific to this chapter and is not consistent with the usual use of the term or with its definition elsewhere in the report.

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100 TABLE 3 . 5 Low-weight Births per 1, 000 Live Births in Relation to Race in the United States and Five States, 1971, 1976, and 1981 White 1971 1976 1981 Black 1971 1976 1981 United States 66 61 57 133 129 125 Californian 51 46 44 111 108 101 Massachusetts 65 63 55 113 125 111 Michigan 64 61 57 144 140 137 North Carolina 70 64 61 135 130 124 Oregon 56 52 47 118 127 103 NOTE: Data presented in this table are derived from the sources descr ibed in reference no. 1. aMultiple live births excluded. Data for white births exclude those with Spanish surnames. For Spanish surnamed births, rates per 1,000 were 53 in 1971, 47 in 1976, and 45 in 1981. relative risks are found among births to teenage mothers, particularly among whites. However, a much greater proportion of black births are to teens. The gap in low birthweight rates between the high- and lower isk maternal age groups shifted only slightly from 1971 to 1981. No particular age group displayed a marked departure from the general pattern of small reductions in the low birthweight rate dur sing the decade. Analysis of the individual state data produced a s impalas conclusion. Table 3.7 shows trends in the relative risk of low birthweight according to maternal educational attainment for whites and blacks separately from 1971 to 1981. For both races, the relative risk of low birthweight declines sharply among mothers with at least 12 years of education. Moreover, the increased r Ask among mothers with less than 12 years education is more pronounced for whites. The relationship between education and low birthweight rates prevails among individual maternal age groups (Appendix Table B.91. There is no indication of a narrowing gap In relative r isk among births to mothers with disparate educational attainment. The trend, in fact, is toward a widening gap. Because the number of states reporting educational attainment varied during the 1971-1981 period, such a f inding requires caution. Generally consistent findings, however, were obtained from analysis of individual selected states (Appendix Table B.10~. Given the overall improvement in levels of educational attainment of mothers from 1971 to 1981, the finding of an increasing gap suggests that those who have remained poorly educated constitute an increasingly high-risk group.

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101 TABLE 3.6 Relative Risk of Low Birthweight by Age of Mother and Race: united States, 1971, 1976, and 1981 Relative Riska Percent Distribution White Black Total Live Births (1981) Maternal Age (years) 1971 1976 1981 1971 1976 1981 White Black Less than 15 1.95 1.91 1.84 1.43 1.31 1.33 O.1 0.9 15-19 1.26 1.32 1.36 1.13 1.17 1.12 12.7 24.4 15-17 -- -- 1.54 -- -- 1.17 4.2 10.5 18-19 -- -- 1.27 -- -- 1.09 8.6 13.9 20-24 0.93 0.98 1.02 0.96 0.97 1.00 33.3 35.4 25-29 0.91 0.86 0.88 0.88 0.87 0.91 32.5 23.7 30-34 0.98 0.94 0.88 0.89 0.89 0.90 16.7 11.5 3S-39 1.20 1.14 1.10 1.02 1.01 0.99 4.0 3.4 40 or older 1.56 1.37 1.31 0.97 1.01 1.07 0.6 0.7 NOTE: Data presented in this table are derived from the sources described in reference no. 1. aThe base for calculating relative risks is the low b~rthweight rate for the total in each column. Table 3.8 reports trends in the relative risk of low birthweight by marital status, maternal age, and race in the United States from 1976 to 1981. Unmarried mothers have a consistently higher risk of bearing a low birthweight infant than those who are married. The increased risk is not explained by age differences among married and unmarried women. The relative risk differential is more pronounced for whites than blacks; but the much larger proportion of unmarried mothers among blacks makes unwary fed status a far more important risk factor for blacks than for whites. Of even greater concern is the fact that births to unmarried women increased throughout the 1970s. Whereas 7 percent of white mothers and 51 percent of black mothers were reported unmarried in 1976, the proper tions were 12 percent for whites and 56 percent for blacks in 1981. It is difficult to discern a clear trend in relative risks from Table 3.8. Such a trend conceivably could be masked by changes in the number of reporting states and the relatively short period examined. For two selected states, Table 3.9 presents trends in low birthweight rates in relation to a simple two-way classification of soc~odemographic risk factors. ~High-risk. mothers are defined as those who reported less than 12 years education, were unmarried, or were young {less than 20 years of age for Massachusetts, less than 18 years of age for North Carolina), or any combination of these risk factors. "Low-risk n mothers include all others. The data show that

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1 102 TABLE 3.7 Relative Risk of Low Birthweight by Educational Attainment of Mother and Race: 1971, 1976, and 1981 Relative Riska Maternal . Percent Distribution Educational White Black Total Live Births (1981) Attainment (years) 1971 1976 1981 1971 1976 1981 white Black 0-8 1.35 1.39 1.46 1.08 1.14 1.17 9-11 1.32 1.35 1.47 1.10 1.12 1.18 12 0.92 0.94 0.96 0.92 0.92 O.95 13-15 0.80 0.82 0.81 0.81 0.85 0.83 16 or more 0.74 0.73 0.74 0.74 0.72 0.71 3.7 4.7 15.8 30.1 44.0 41.5 18.9 15.5 16.2 6.5 NOTE. Includes 49 states and Washington, D.C., in 1981; 44 states and Washington, D.C., in 1976; 38 states and Washington, D.C., in 1971. Data presented in this table are derived from the sources described in reference no. 1. aThe base for calculating relative risks is the low birthweight rate for the total in each column. declines in low birthweight rates have been concentrated in ache lower isk groups . Such evidence supper ts the conclus ion that the gap in low birthweight rates between low-risk and high-risk groups, at least as defined by maternal sociodemographic characteristics, is not closing. Pregnancy History Birth order is associated with smaller differentials in relative r isk than is age, and here again the relative r isk var ies ~ ess among blacks than whites (Table 3.10 and Appendix Table B.ll). The changes in relative r isk have been small. Still, among whites there is a suggestion that first births are at increased relative r isk for low bir thweight. The effect of the interaction between maternal age and birth order on low birthweight has been well documented in the past and these relationships persist to the present. Figure 3.1, based on 1981 data, indicates the high r isk among women aged 15-19 bear ing their second or later child; the sharp increase in risk among women having their first child as age advances past 25-29 years; and the lowered r isk at third or higher birth orders when women are 25-34 years of age. Very similar patterns are found among wh, te and black women. Termination of last prior pregnancy with a fetal death elevates risk for low birthweight in the next pregnancy, but the increase is not dramatic (Table 3.10~. The relationship is stronger for whites than

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103 TABLE 3.8 Relative Risk of Low Birthweight by Marital Status, Age of Mother, and Race: 1976 and 1981 Relative Riska Percent Distr ibution White Black Total Live Births (1981) 1976 1981 1976 1981 White Black Married 0.97 0.92 0.87 0.81 88.4 44.1 Unmarried 1.60 1.57 1.14 1.15 11.6 56.0 Marr fed Less than 15 2.07 1.70 1.55 1.50 b ~ 15-19 1.25 1.26 1.06 0.97 8.3 3.4 15-17 - - 1.45 -- 1.07 2.2 0.7 18-19 - - 1.17 -- 0 .94 6 .1 2.7 20-34 0.90 0.88 0.83 0.79 75.7 37.8 35 or older 1.36 1.06 1.02 0.87 4.4 2.9 Unmar r fed Less than 15 1.89 1.88 1.32 1.33 0.1 0.9 15-19 1.59 1.58 1.16 1.14 4.4 21.0 15-17 -- 1.61 -- 1.18 2.0 9.9 18-19 -- 1.54 -- 1.12 2.4 11.2 20-34 1.66 1.56 1.12 1.14 6.7 32.8 35 or older 1.97 1.82 1.13 1.32 0.3 1.2 NOTE: Includes total united states in 1981, 38 states and Washington, D.C., In 1976. Data presented in this table are derived from the sources described in reference no. 1. aThe base for calculating relative r isks is the low birthweight rate for the total In each column. bless than 0 .1 percent. blacks. Differentials in relative risk between births preceded by a live birth and those preceded by a fetal death have decreased. The data related to the outcome of the last pregnancy need to be interpreted cautiously, however, because of likely problems in reporting accuracy and consistency, and variations in geographic coverage. Still, the narrowing of the gap observed is one of the few evidences of positive change in relative risk during the 1971-1981 period. Special tabulations on 1981 live births, performed by the National Center for Health Statistics for this report, showed the relationship of birthweight to interval between termination of ~h`? ~ A=F nr-"n~n^v and the first day of the last menstrual per con before the current pregnancy. Analysis was confined to single births in which the prior pregnancy ended in a live-born infant. Table 3.11 shows a sharply . . . ~ = _ _ ~ ~ _,! . eleValea EelaLlVe ~1SK arena heath rant then Who i"~',=1 i- 1~- -~_ 6 months. (Bowever, only 5-10 percent of births fail into this inter- pregnancy interval.) Risks decrease moderately thereafter to reach an optimum level at 2 to 3 years after the previous pregnancy in the white group and 2 to 4 years in the black group. The relative risk increases for births with longer interpregnancy intervals, especially in whites, reflecting, perhaps in part, impaired fecundity. Although the data are not shown here, the pattern of exceptionally high relative risk for a ~ _ = ~ ~ ~~ _~. _~ ~ 44 ~^ or ~ ^ ~ =~ ~1

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104 TABLE 3 . 9 Trends in Low Birthweight Rates in Relation to Two-Way Classification of Maternal Risk, Massachusetts and North Carolina, 1971, 1976 , and 1 981 . 1971 1976 1981 Massachusetts Higher isk 93 8 9 8 7 Lower isk 61 57 49 Nor th Carol ina Higher isk 10 9 10 6 10 6 Lower isk 62 58 55 NOTE: ~High-risk. births defined as either maternal education less than 12 years, mother unmarried, or young maternal age (less than 20 for Massachusetts, less than 18 for North Carolinas, or any combination of the above. Lower isk. includes all others. Data presented in this table are der iced from the sources descr ibed in reference no. 1. TABLE 3.10 Relative Risk of Low Birthweight by Birth Order, Outcome of Prior Pregnancy, and Race: United States, 1971, 1976, and 1981 Relative Riska Percent Distribution White Black Total Live Births tl981) 1971 1976 1981 197' 1976 1981 White Black Birth Order First birth 1.01 1.06 1.08 1.02 0.85 1.00 43.7 39.2 Second birth 0.93 0.89 0.90 1.01 0.98 0.96 32.5 29.3 Third birth 0.98 0.97 0.94 1.00 0.97~ 0.99 14.7 16.6 Fourth birth 1.09 1.10 1.08 0.99 1.02 1.06 5.2 7.6 Fifth and over 1.17 1.14 1.11 1.05 0.98 1.12 3.5 9.9 Outcome of Prior _ I_ . . PregnancyD Live birth O.95 0.94 0.92 0.95 0.94 0.95 75.6 72.6 Fetal death 1.31 1.32 1.21 1.32 1.28 1.11 17.8 17.0 Unknown 1.37 1.38 1.30 1.31 1.22 1.17 6.5 10.S NOTE: Data presented in this table are derived from the sources described in reference no. 1. aThe base for calculating relative risks is the low birthweight rate for the total in each column. bData are for second order births or higher. States reporting the information varied: 49 and Washington, D.C., in 1981; 43 and Washington, D.C., in 1976; 37 and Washington, D.C., in 1971.

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105 20 18 16 _ 14 - o o '_ 12 At c' 1 0 OR us 8 6 4 2 o 20 18 t6 14 o 12 U] C) 10 8 6 4 2 o ~[ 3rd and 4th \ 2nd \ \ \ ; 1st ~ ~ Huh and Over ~ ~ ALL RACES - 1 1 1 1 1 5-1 9 20-24 25-29 30-34 35+ WE ITE 3rd and 4th \ 5th and Over 2nd \ 1st\ \ N~ \ i' - - - 1 1 1 1 J 15-19 20-24 25-29 30-34 35f 20 3rd and 4th 18 In>\ " Sth and Over \ \ " ~ \ "` ~ ~ 1st \ \ _ ~ ~ 16 14 12 U] OCR for page 94
106 TABLE 3.11 Relative Risk of Low B~rthweight by Interval Between Last Pregnancy and Current Pregnancy (Single Live Births), 1981 Relative Risks Percentage of All Live Births Interval (months) White Black White Black Less than 6 1.63 1.46 5.4 8.9 6-11 1.04 1.06 12.1 13.2 12-23 0.81 0.92 24.2 18.9 24-35 0.78 0.83 15.1 11.7 36-47 0.86 0.83 8.9 8.4 48 or more 1.07 0.90 17.6 23.0 Unknown 1.24 1.13 16.7 15.9 NOTE: Includes only births in which the last prior pregnancy terminated in a live birth. Pregnancy measured from first day of last menstrual period. Reporting areas include 49 states and Washington, D.C. Data presented in this table are derived from the sources described in ~ reference no. 1. aThe base for calculating relative risks is the low birthweight rate for the total in each column. short interval between pregnancies and of declining relative risk with increases in the interpregnancy interval is found at every age and birth order. An independent effect of interpregnancy interval on birthweight remains apparent even after controlling for the potentially confounding variables of age, birth order, race, marital status, and educational attainment. The relationship between birthwe~ght and interpregnancy interval is much different for pregnancies where the previous pregnancy terminated in ~ fetal death. A short interval between pregnancies is not associated with increased risk of low birthweight. In fact, the reverse is suggested. Prenatal Care The relationship between the risk of low birthweight and the timing and quantity of prenatal care is reviewed in detail in Chapter 6. Table 3.12 and Appendix Table B.12 provide information on this issue drawn from vital statistics. Among whites, there is an increase in relative risk as the time of the first prenatal visit advances from the first and second month of pregnancy to the second trimester. AS other investigators have noted, 3 the risk of low birthweight appears reduced for mothers who begin care in their third trimester because such pregnancies already have reached more advanced gestational ages. Despite this bias, the relative risk remains high among the whites who

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107 TABLE 3.12 Low Birthweight Relative Risk by Month Mother Began Prenatal Care and Race: 1971, 1976, and 1981 Relative Riska - Percent Distr ibution Start of White Black Total Live Births (1981) Prenatal Care (month) 1971 1976 1981 1971 1976 1981 White Black First-second 0.92 0.90 0.89 0.93 0.91 0.92 55.0 38.6 Third 0.94 0.95 0.95 0.93 0.97 0.94 24.4 23.9 Fourth-sixth 1.12 1.15 1.16 0.96 1.00 1.01 16.3 28.5 Seventh-ninth 1.08 1.10 1.12 0.86 0.93 0.94 3.2 6.2 No prenatal care 2.75 2.69 2.88 2.13 2.22 2.21 1.1 2.8 Seventh month or later or no care 1.42 1.48 1.56 1.24 1.31 1.35 4.3 9.1 Four th month or later or no care 1.17 1.20 1.23 1.04 1.06 1.09 20.6 37.6 NOTE: Includes total U.S. in 1981; 44 states and Washington, D.C., in 1976; 39 states and Washington, O.C., in 1971. Data presented in this table are derived from the sources descr ibed in reference no. 1. aThe base for calculating relative r isks is the low bir thweight rate for ache total in each column. began care after the sixth month. For the no-care group, by contrast, many of the pregnancies may have terminated early, before such women would ordinarily have started prenatal care. Analysis of trends in low birthweight rates in relation to prenatal care is complicated by variations over time in the number of states reporting data on such care. From reporting states (see Appendix Table B.12) , it appears that low birthweight rates for all races combined declined in both early- and late-care groups, though the low birthweigh. rate for mothers with early care improved relatively more than the other s . For the individually selected states, analysis of the relationship between low birthweight rates and the timing of the first prenatal visit gave s imilar r esults (Appendix Table B.13~. Except in North Carolina, the great bulk of the decline in low birthweight rates has occurred among the early-care group. It is noteworthy that substantial increases occurred during the 1970s in the proportion of mothers reporting a first visit during the first trimester. Thus, for Massachusetts, mothers with early care comprised 79 percent of live births in 1970 and 87 percent of live births in 1980. For Michigan, the corresponding proportions for 1970 and 1980 were 69 percent and 78 percent, respectively. For North Carolina, the proportions were 67 percent and 78 percent, respectively; and for Oregon, the proportions were 71 percent in 1971 and 77 percent in 1980. For reporting states in the United States as a whole, the proportions with first trimester care were 79 percent of whites and 63 percent of blacks by 1980. The following year there was no decrease and in 1982, the figures were 79 and 61 percent, respectively, representing a departure from the steady increases during the decade 1970 to 1980.

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108 Analysis of 1981 Single Live Births in the United States* This section explores in more detail the relationship between low b~rthweight and certain maternal characteristics. The objective is to assess what fraction of current low-weight births might be eliminated by improvements in prenatal care and specific risk factors. This analysis ignores the finding of previous sections that past reductions in low birthweight were in some cases more pronounced among low-risk groups. The upper part of Table 3.13 shows, for whites and blacks sepa- rately, the estimated effect on the 1981 low birthwe~ght rate of eliminating all late prenatal care and the associated excess risk, as well as the estimated effect of eliminating late prenatal care and improving selected maternal risk factors (less than 12 years education, unmarried status, and high age of mather/b~rth order (ABO) risk, as defined in Table 3.13~. The lower part of the table shows the same analysis, except that elimination of "nonadequate" care, rather than late care, is considered. "Late prenatal care" is defined as care begun after the first trimester of pregnancy or no care. "Nonadequate prenatal care. means either late start of prenatal care or a first trimester start with fewer prenatal visits than prescribed for the reported duration of pregnancy (an index adapted from the one developed by Ressner, et al .4 ~ . The estimated effects in Table 3.13 correspond to what epidem~olo- gists term "attributable risk. reduction; that is, they reflect both the relative risks of low birthweight among the maternal risk categories and the proportions of single live births in 1981 among the risk categories. As the upper section of the table shows, late prenatal care by itself is a relatively minor factor in explaining the level of the low birthweight rate in 1981 for both racial groups; social and pregnancy history factors are far more important. The quantitative contribution of prenatal care increases appreciably when the measure of care takes into account the frequency of prenatal visits as well as the timing of the first visit (lower part of table). Eliminating excess risks among births with "nonadequate" care reduces the low birthweight rate by 15 percent among whites and 12 percent among blacks, in contrast to a 3 percent reduction for both races when prenatal care is gauged only by whether care started in the first trimester. The joint effect of eliminating nonadequate prenatal care and the other characteristics is substantial. It is recognized that the preceding analysis controlled for only a limited number of risk factors. From the vital record data alone, the content of prenatal care could not be examined. Still, the results in Table 3.13 suggest that the pattern of prenatal care, rather than early initiation of care alone, may play an important role in determining birthweight. *Based on special tabulations provided by Dr. Joel Rle~nman, National Center for Health Statistics.

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109 TABLE 3.13 Estimated Effects on Low Birthweight of Improvements in Prenatal Care and in Selected Maternal Risk Factors (Single Live Births), 1981 white Black Rate Percent Rate Percent (per 1,000) Reductiona (per 1,000) Reductions Original base rate 47 -- 112 Estimated rate after elimination of excess risk among births with:b Late prenatal care 46 3 107 __ 3 Late prenatal care and less than high school education, unmarried status, or high ABO risks 38 20 84 24 Estimated rate after elimination of excess risk among births with:b Nonadequate prenatal care 40 15 97 12 Nonadequate prenatal care and less than high school education, unmarried status, or high ABO risks 34 29 77 30 NOTE: United States, excluding births in California, New Mexico, Texas, and Washington, which did not report mother's education or number of prenatal visits. Data presented in this table are derived from the sources described in reference no. 1. aPercent reduction determined from rates calculated to additional significant digit. stimated rates are derived by adjusting data on live births cross-tabulated by the specified variables to eliminate associated excess risks. CHigh ABO risk includes births to women under 18 years of age, second or higher order births to 18- and 19-year-old women, first order births to women 30 years of age or older, and all other births to those age 40 and older.

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110 Conclusions The committee investigated trends in the rate of low birthweight and the composition of low-weight births in the United States as a whole and in five selected states (California, Massachusetts, Michigan, North Carolina, and Oregon) during the past 10 to 15 years. For the United States, the proportion of low-weight births declined from 7.6 percent of live births in 1971 to 6.8 percent of live births in 1981. Although Massachusetts showed a more marked decline in low birthweight rates than the other states, the relative declines in all of the states and the United states were of the same order of magnitude (Table 3. 1~ . There is no clear indication that low birthweight rates were declining more rapidly during the earlier or later part of the 1968-1982 period. Moreover, inclusion of fetal deaths in the analysis of low birthweight trends results in only a slight to moderate increase in the rate of low birthweight decline (Table 3.2~. The decline in low birthweight rates was confined to the moderately low birthweight group (1,501-2,500 grams). No decline, or perhaps a slight increase, was observed in the very low birthweight group (1,500 grams or less) (Table 3.3~. Although birth certificate data on gesta- tional age are incomplete and of uncertain quality, the observed decline in low birthweight was apparently concentrated mostly in the full-term low birthweight group. However, in Massachusetts and Oregon, where overall declines in low birthweight rates were larger, a decline in preterm low birthweight rates also was observed. In each state, among moderately low-weight births, most of the decline was observed among term infants (Table 3.41. Both white and black low birthweight rates have declined, but blacks remain at increased risk of low birthweight compared to whites. The data show no closing of the gap, although quantifying the white-black differential in low birthweight rates depends on the index used to measure the gap. Both white and black low birthweight rates vary appreciably across the states studied individually. Teenage mothers and those aged 35 years or over have higher risks of low birthweight than mothers in their twenties or early thirties. Teenage mothers have the highest relative risk of low birthweight, especially among whites (although this elevated risk probably derives more from other characteristics of teenage mothers than from young age itself, as discussed in Chapter 2~. Childbearing among teenagers is more prevalent among blacks, however. In both races, no per titular age aroun showed a marked departure from the general pattern of small _ ~ . _ _ _ ~ ~ , _ reductions in low birthweight from 1971 to 1981 (Table 3.61. For both races, the risk of low birthweight declines sharply among mothers with at least 12 years of education. The relationship between education and low birthweight is independent of maternal age and race. The gap in low birthweight rates among mothers with disparate educational attainment is not closing, and may be widening. Because educational attainment of mothers has increased during the past 10 to 15 years, the finding of a widening gap in low birthweight among mothers with disparate education suggests that the poorly educated constitute an increasingly high-risk group (Table 3.7~. may

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111 Unmarried mothers have consistently higher risks of low birth- weight. The elevated risk is not attributable to differences in age or race. No clear change in the low birthweight differential between married and unmarried births has been observed. However, the propor- tion of unmarried mothers appears to be increasing. Among blacks, 56 percent of mothers were reportedly unmarried in 1981 (Table 3.8~. A two-way measure of maternal sociodemographic risk was devised to reflect maternal educational attainment, marital status, and age. An analysis of trends in Massachusetts and North Carolina showed that the gap in low birthweight rates between high- and low-risk groups has been widening (Table 3.9~. There has been no clear change in the relationship between par ity and the risk of low-weight birth. High-parity births continue to have slightly higher risks of low birthweight (Figure 3.1~. Moreover, termination of the last pregnancy with a fetal death increases the risk of low birthweight in the subsequent pregnancy. However, the differential in low birthweight between those with a prior live birth and those with a prior fetal death appears to have narrowed (Table 3.10~. Among mothers with a previous live birth, an interpregnancy interval of less than 6 months enhances the subsequent risk of low birthweight (Table 3 .11 ) . The r isk of low birthweight is reduced among mothers who initiate prenatal care dur ing the first 3 months of pregnancy. The proportion of mothers with early prenatal care increased during the past 10 to 15 years. There was also a decline in low birthweight rates among mothers In the early care group (Table 3 .12 ~ . The committee performed a multivariate tabulation of single live births in the United States during 1981 according to educational at- tainment, marital status, age/b~rth order category, and the timing and quantity of prenatal care. With other factors controlled, a change in only timing of prenatal care starts was associated with a minor reduction in risk of low birthweight. The estimated contribution of prenatal care to a reduction in the low birthweight rate was more marked, however, when care was gauged by an index of "adequacy" that reflected both the start of care in the first trimester and number of visits. The tabulation suggested that the analyzed risk factors might account together for as much as 30 percent of the risk of low birth- weight in both races. Moreover, the sensitivity or the estimated contribution of prenatal care to the method of measuring such care suggests that the pattern of care may be an important influence on the risk of low birthweight (Table 3.131. Chapter 6 explores this theme more fully. In reaching these conclusions, the committee identified several issues for future analyses of low birthweight using vital statistics. First, as noted also in Chapter 2, pregnancy outcome measures need to be defined more precisely--e.g., very low birthweight, moderately low birthweight and preterm and full-term low birthweight. And differen- t~al rates among white and black mothers, among those with higher and lower educational attainment, and among other high-risk and low-risk groups deserve careful scrutiny. me,

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112 Future sources of information on low birthweight and other preg- nancy outcomes also need to be more timely. Available data collection and reporting procedures, as well as terminology, need to be more uniform across states. vital record data alone may not gauge adequately the content and effectiveness of prenatal care; nor do such data character ize important aspects of maternal behavior (such as cigarette smoking) and medical history {such as maternal diabetes). Accordingly, high priority ought to be given to more detailed studies of selected cohorts of pregnant women. These should include new prospective designs and reanalysis of existing data. Further study is required on methods to measure more precisely the content and timing of prenatal care. Greater attention should be given to analyses of the experiences of individual states and other jurisdictions. Geographic var. iations in pregnancy outcome or in the relationship between low birthweight and other r isk factors could reflect differences in local public health programs and economic activity, as well as sociodemographic variations. In making these observations, the committee recognizes that it is giving only limited attention to important problems In the nature, quality, and timeliness of the vital statistics data collected and analyzed in the United States. Limits of time and resources prevented the formulation of detailed suggestions in this area, aside from those above, and the committee's charge did not require an ~n-depth review of data collection issues. Nonetheless, Appendix D describes some important national vital statistics data sets that can help in studying the problem of low birthweight. The committee is also aware that several groups are already concerned with vital statistics issues (such as the National Committee on Vital and Health Statistics, the National Association of Vital Registrars, and the ad hoc Standard Terminology Group), and urges such groups to address the data problems highlighted in this chapter, Chapter 6, and elsewhere in the report. References and Notes 1. All data for the United States are from published and unpublished data provided to the committee by the National Center for Health Statistics. For California, Michigan, and Oregon, the analyses were derived from cross-tabulations performed by organizations in the respective states. For Massachusetts and North Carolina, the cross-tabulations were performed by the committee from public use tapes. Ressel SS, Villas J. Berendes HW, and Nugent RP: The changing pattern of low birthwe~ght in the United States: 1970-1980. JAMA 251:1978-1982, 1984. 3. Harris JE: Prenatal medical care and infant mortality. In Economic Aspects of Health, edited by VR Fuchs, pp. 15-52.- Chicago: University of Chicago Press, 1982. 4. Institute of Medicine: Infant Death: An Analysis by Maternal RiSk and Health Care. Contrasts in Health Status, edited by DM Ressner, Vol. . 1. Washington, D.C .: National Academy of Sciences , 1973 .