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OCR for page 63
4
Assessment of
Gestational Weight Gain
In this chapter, methods and issues related to the assessment of mater-
nal weight gain during pregnancy are discussed. The types of data essential
to assessment of gestational weight gain in the clinical setting are identified,
and desirable methods of collecting, using, and interpreting these data are
described. The subcommittee then discusses various definitions of gesta-
tional weight gain and other factors that have complicated interpretation
of the research on health implications of gestational weight gain.
METHODOLOGICAL ISSUES RELATED TO
DATA COLLECTION IN THE CLINIC
When weight and weight gain are used to screen individual women
for special treatment or to monitor the progress of a pregnancy, the
consequences of misdiagnosis resulting from poor data quality may be
substantial. Therefore, major emphasis should be placed on establishing
and implementing data collection techniques that provide the most reliable
and accurate data possible for the particular clinical setting.
Clinical data relevant to gestational weight gain include weight, height,
and gestational age. Other measures, such as skinfold thickness and indica-
tors of frame size, are also occasionally recommended but are not included
in this critique. For reasons discussed in Chapter 6, the measurement
of skinfold thickness to assess composition of weight gain and nutritional
status during pregnancy has not been thoroughly studied or validated.
Similarly, there have been no studies on pregnant women to validate the
63
OCR for page 64
64
NUTRITIONAL STATUS AND WEIGHT GAIN
functional meaning of frame size indicators (e.g., elbow breadth and wrist
circumference or diameter measurements suggested as a basis for improv-
ing weight-for-height classification schemes). Therefore, the subcommittee
believes it is premature to recommend specific techniques for making these
measurements. Before making any of the measurements described below,
one should consult a reliable source such as Lohman et al. (19~) for a
thorough review of methods and techniques.
Weight
Weights measured in the clinic are preferable to self-reported or re-
called weights. However, measurement alone does not ensure accuracy.
Factors that can affect measurements of a woman's weight include the type
of scale used (e.g., spring or beam balance scale), the accuracy of its cali-
bration, the clothing worn by the woman, the time of day the measurement
is made, and the contents of the bowel and bladder at that time. When
considering efforts to improve the reliability of measurements, it is useful
to keep in mind their purpose. Control of errors is most crucial when
weights are collected over a short period (e.g., 1 to 2 weeks). For example,
normal weight gain during the second trimester is approximately 0.4 to 0.5
kg (~1 lb) per week, or 1.6 kg (3.5 lb) per month, but it is not unusual for
the accumulated errors in weekly or monthly weighings to total 0.4 to 0.7
kg (1 to 1.5 lb) in many clinics. Since a subnormal gain is often defined as
less than 1 kg (2 lb) per month, with this degree of error, it might be very
difficult to distinguish abnormal weight gains from measurement error over
relatively short intervals.
Subjects should be weighed on a platform beam balance scale with
movable weights or on a high-quality electronic scale. The scale should be
graduated to the nearest 100 g (0.25 lb) and calibrated periodically against
a known weight or series of weights approximating the range of weights
encountered in clinic patients. For example, barbell weights or boxes of
books with known (recorded) weights can be stacked to cover the range of
calibration weights. Subjects need not be weighed while they are nude, but
procedures should be followed to promote consistency in the amount of
clothing worn by all patients. The fewer clothes the better, as long as the
weight of the clothing can be kept relatively constant for all women and all
seasons of the year. This requires weighing women without purses, shoes,
boots, coats, jackets, or any accompanying young children. All personnel
responsible for taking weights and other measurements should be trained
in the standard procedures chosen by the clinic. It should not be assumed
that everyone knows how to obtain weight measurements properly. For
clinical management of the patient, it is most important to have accurate
measurements taken sequentially throughout the pregnancy.
OCR for page 65
ASSESSMENT OF GESTATIONAL WEIGHT GAIN
65
Prepregnancy weights are determined primarily to establish whether
the patient's weight is high, low, or normal and to provide a basis for
identifying extreme first-trimester weight changes. Prepregnancy weight is
best determined from objective data from the patient's chart reflecting a
recent preconceptional visit. Self-reported Prepregnancy weights must be
evaluated for plausibility and discarded if they are suspect.
Height
Knowledge of standing height (stature) is indispensable when careful
classification by Prepregnancy weigh-l status is desired. Such classifications
are based on body mass index (BMI; see the section Assessing Prepregnancy
Weight for Height below) or on a percentage of a standard reference weight
for a given height, e.g., the Metropolitan Life Insurance (MLI) Company
standards for ideal body weight (1959, 1983~. Height can be measured with
reasonable precision in a clinical setting if standard procedures are followed.
A low-cost stadiometer can be constructed from a metal measuring tape
and a headboard. The tape is attached to a vertical wall against which
the patient can stand erect with the back, buttocks, and heels against
the tape. The headboard can be constructed from two pieces of wood
joined to form a right angle. The vertical board is pushed flat against
the wall and lowered along the tape measure until the horizontal board
makes contact with the patient's head. The measurement is taken at the
point where the horizontal board touches the patient's scalp. Commercially
available stadiometers can also be used, but the ones built into weighing
scales are unsuitable for research purposes. The stadiometer should be
inspected periodically to ensure that its position has not changed with use.
Height should be measured as early in the pregnancy as possible to avoid
errors caused by body posture changes, which may affect measurements
beginning approximately 20 weeks into gestation. Postural change may
reduce measured height by approximately 1 cm (0.4 in.~.
Gestational Duration
The adequacy of accumulated weight gain or achieved weight status
at any given stage of pregnancy can be determined only when the length
of gestation is known. An accurate estimate of length of gestation is also
essential for research on gestational weight gain and fetal growth. By far
the most common method of estimating length of gestation is calculation
of the time since the last menstrual period (LMP) based on dates provided
by the woman at her first prenatal visit. There is an established literature
(e.g., Oates and Forrest, 1984; Wilcox and Homey, 1984) dealing with the
accuracy of recall of this date. Recall becomes increasingly problematic as
OCR for page 66
66
NUTRITIONAL STATUS AND WEIGHT GAIN
the LMP becomes more distant in time and memory becomes less reliable.
Also, a small percentage of conceptions in the United States occur during
amenorrheic cycles. In some pregnancies, vaginal bleeding occurs within
the first 4 to 6 weeks after conception and is falsely reported as a menstrual
flow. Moreover, the date of the LMP does not denote the beginning of
the pregnancy but only the presumed beginning of the cycle that produced
the ovum that was fertilized 10 to 14 days later. Actual gestational length,
or time from conception, is rarely known and is even less likely to be
accurately self-reported several months after conception. Actual length of
gestation is generally calculated from the estimated date of conception up
to the date of the prenatal exam or delivery, whichever is applicable.
In the clinic, a gestational age calculator or table, special computer
program, or programmed hand calculator should be used to estimate the
length of time since last menstruation, rather than calculations made in the
head or on paper. Early (prior to 20 weeks) ultrasound measurement of the
fetal biparietal diameter represents an alternative for estimating gestational
duration.
Although errors in estimating gestational duration may pose problems
of interpretation when one uses incremental weight gain from conception
to a point later in gestation, they do not do so when estimating the rate
of weight gain between two or more prenatal visits after the first trimester.
A shift of several weeks in the gestational age estimate does not affect the
interpretation of the rate of gain, since it is generally linear over a broad
range of pregnancy from 13 to 35 whelp of gestation.
Assessing Prepregnancy Weight for Height
~ assess prepregnancy weight for height, it is necessary either to
calculate BMI or to compare the weight for height to a weight-for-height
reference standard (e.g., MLI ideal weight tables). ~ minimize the chance
of calculation errors, it is advisable to use the units of measure used in the
clinic (i.e., metric or English system).
BMI is the most common expression of body weight corrected for
height in use today. It is computed as follows:
BMI = ht2 = g2 x 1~, or i 2 x 1~.
BMI can be easily determined from the table in Appendix C. A
similar weight-for-height index is achieved by expressing body weight as a
percentage of desirable body weight adjusted for height in the following
form:
OCR for page 67
ASSESSMENT OF GESTATIONAL WEIGHT GAIN
Percentage of reference weight = observed weight x 100
expected or desirable weight
given the patient's height
67
The most widely used values for bracketing normal weight seem to
be 90 and 120% of the 1959 MLI (Metropolitan Life Insurance Company,
1959) reference weights. This, in turn, corresponds to a metric BMI of
approximately 19.8 to 26.0. However, some clinicians use 80% of the
1959 MLI reference as the cutoff point for distinguishing those who are
underweight, while others may use 95% as the cutoff point. The cutoff
point for overweight may be as low as 110% to as high as 130% of MLI
reference weights. Moreover, an increasing number of clinics are using the
1983 version of the MLI reference weights (Metropolitan Life Insurance
Company, 1983~.
There is no statistical or scientific basis for prescribing one set of cutoff
values or reference standards over another when assessing prepregnancy
weight. The relationship between prepregnancy weight and various fetal
or maternal outcomes is generally considered to be linear, with no well-
defined threshold at either end of the prepregnancy weight distribution,
but this view has not been verified by research. Pending further research,
the subcommittee recommends using a metric BMI of 19.8 to 26.0 as
the definition of normal (moderate) prepregnancy weight for height. This
BMI range (see Appendix C) encompasses the 25th to 75th percentiles of
prepregnancy weight for height of women in the 1980 National Natality
Survey (Kleinman, 1990~.
BMI is a preferred indicator of nutritional status because it depends
on two commonly measured aspects of body morphology-weight and
height. BMI is often viewed as a proxy for more accurate indicators of
body composition such as body fat content or lean body mass, but it is
not simply an obesity index. Women with BMIs greater than 26.0 do not
necessarily have excess body fat deposition; in some, the extra weight may
be attributable predominantly to muscle tissue or skeletal mass. Studies
relating BMI or its equivalent to estimates of body fat obtained from more
complex methods, such as those described in Chapter 6, have indicated
a good correlation between percentage of body fat and BMI (e.g., R =
.7; Jackson et al., 1988), but there will be considerable misclassification of
undernutrition or obesity.
Using Weight Gain Data in the Clinic
It is desirable to plot cumulated weight gain (weight gain to date)
sequentially on a weight gain chart that has a reference curve that reflects
OCR for page 68
68
NUTRITIONAL STATUS AND WEIGHT GAIN
normal weight gain. This allows visual inspection of the overall trajectory
or slope of the patient's weight gain and comparison with the standard.
It thus provides the clinician with a visual impression of whether or not
weight gain is progressing normally for the mother. Award the end of
gestation, cumulated weight gain becomes increasingly predictive of birth
weight because the weight contributed by the fetus becomes relatively
greater.
Gestational weight gain can be monitored in a manner similar to
methods recommended for monitoring child growth (see Griffiths, 1985~.
For example, at the third prenatal visit it is useful to compare the rate
of gain between the second and third visits to the rate of gain between
the first and second visits. A rapid weight gain is more acceptable if
the starting weight was unusually low for the woman's height or if the
weight gain between the first and second visits was unusually low. In
both cases, one might interpret the rapid weight gain as catching up to
or compensating for poor earlier weight status. In contrast, if a woman
were obese before pregnancy and she gained weight rapidly between each
successive measurement, there is cause for concern. Unusually high or low
gains should be checked for possible measurement or recording error.
To determine numerically whether the rate of weight gain is close to
that recommended, the following equation can be used:
wet-wit_
weight gain =
where wt is weight, GA is gestational age in weeks, ~ is time of most recent
measurement, and t-1 = time of previous measurement. As an example,
if body weight is 135 lb at week 20 of the pregnancy and 139 lb at week 25,
then
weight gain (lbhvk) = 25 k 20 k = 5 k = 0 8 lb~wk.
It is less useful to calculate rate of gain for periods including part or
all of the first trimester, when the rate of gain is normally low, than for
periods within the second and third trimesters.
Weight Gain Charts
Many different weight gain charts and tables are available for use in
the clinical assessment of gestational weight gain; Table 4-1 presents se-
lected examples representing a wide range of applications. An evaluation
of the characteristics of these various charts is useful in considering their
application in the clinical setting. Weight gain charts evolved over the past
OCR for page 69
ASSESSMENT OF GESTATIONAL WEIGHT GAIN
69
40 years in four basic stages: (1) early, normative charts constructed from
a homogeneous sample (usually a very small sample) of healthy women
who delivered normal infants, (2) modification of the early normative
charts to account for maternal prepregnancy weight, (3) elaboration of the
charts that incorporate prepregnancy weight status by inclusion of more
recent normative data on gestational weight gain, and (4) elaboration of
prepregnancy weight classification to establish target weights based on some
percentage of the ideal or reference prepregnancy weight. However, there
is still a need for a validated chart, with the characteristics outlined in
Appendix ~
In Table 4-1, the large number of charts from the 1980s reflects an
increased interest by the scientific community and a growing awareness
of the inadequacy of early weight gain charts among practitioners. Also,
the expansion of prenatal services in the Supplemental Food Program for
Women, Infants, and Children (WIC) created a demand for gestational
weight gain charts. The subcommittee had access to weight gain charts
used by WIC in 21 states. They represent a broad range of different types
of charts, based on different sources of information on normal weight gain.
As described in Table 4-1, most of the charts developed for WIC at the
state level include the normative curve originally reported by Tompkins
and Wiehl (1951) (Figure 4-1) and make some allowance for prepregnancy
weight for height. The mean total gestational weight gain for the 60 women
in Tompkins and Wiehl's sample was 10.9 kg (24 lb), but no ranges, standard
deviations or nercentile.s were renort~`l
~ O
The curio developed by Tompkins and Wiehl has been used differ-
ently in different charts. Some states (e.g., Ohio, Georgia, Oregon, Mas-
sachusetts, New Hampshire, and Rhode Island) use the curve to represent
the average or target weight gain for women with normal prepregnancy
weight. In the chart used by Oregon WIC (Oregon WIC Staff, 1981) (Fig-
ure 4-2), upper and lower limits have been added to the original Tompkins
and Wiehl cube, but their derivation is not given. Other states use different
curves for women who are overweight or underweight before pregnancy
(e.g., Figure 4-3~. Some states (e.g., Wisconsin, Florida, New Jersey, Idaho,
Washington, Wyoming, and New York) use the Tompkins and Wiehl curve
to depict the lower limits of normal. Some use separate charts for normal
weight, overweight, and underweight women, and also include upper and
lower limits, such as those in Figure 4-4A through D. The limits have not
been based on systematic studies of the consequence of misclassification
resulting from the use of specific cutoff points or limits such studies have
not yet been conducted.
Dimperio (1988) has developed a weight gain chart which includes
an 11.4-kg (25-lb) total gain marking the lower limit and a procedure for
adjusting the single curve for different prepregnancy weights for height
OCR for page 70
70
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OCR for page 73
ASSESSMENT OF GESTATIONAL WEIGHT GAIN
28
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Week of Gestation
FIGURE 4-1 Weight gain god from Tompkins and W~ehl (1951~.
73
(Figure 4-5~. Several states (e.g., Indiana, Kansas, Mississippi, and Col-
orado) use weight gain charts based on normative data for a group of
low-income pregnant women from Cleveland and Minneapolis (Brown et
al., 1986) (Figure 4-6~. For these charts, the total target weight gain for
women with a normal prepregnancy weight is 14.3 kg (31.5 lb), which is
approximately 2.9 kg (6.5 lb) higher than specified on most of the charts
mentioned above. Several authors (e.g., Gueri et al., 1982; Husaini et al.,
1986; and Rosso, 1985) have developed single charts or tables to be applied
across a range of prepregnancy weights, heights, or weights for height. A
table of reference weights for height developed by Gueri et al. (1982) is
intended to be used at any stage of pregnancy. Figure 4-7 is based on data
from that table to illustrate the weight gain patterns suggested by Gueri
et al. The basis for and limitations of this table are discussed later in this
chapter.
In order to accommodate inconsistency in the application of propor-
tionate weight gain recommendations across the range of prepregnancy
weight for height, Rosso (1985) developed a weight gain grid that ap-
plies different assumptions of weight gain patterns for underweight and
overweight women (Figure 4-8~. Rosso's grid suggests that normal-weight
women should be advised to achieve 120% of their prepregnancy refer-
ence weight for height but that overweight women (e.g., those weighing
up to 140% of reference weight for height) should gain up to 115% of
their pr~pregnancy reference weight. Rosso's chart, like that of Gueri et
al. (1982), suggests that underweight women gain all their weight deficit
up to their prepregnancy reference weight as well as the additional weight
OCR for page 85
85
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OCR for page 86
86
NUTRITIONAL STATUS AND WEIGHT GAIN
Estimation of Gestational Weight Cain in Research
Information on gestational weight gain is commonly used in biomedical
research, public health, and nutritional surveillance as well as in clinical
care. Each application has its own requirements for the method of ex-
pressing weight gain, but each benefits from rigorous attention to collecting
high-quality data. The following discussion deals primarily with uses of
gestational weight gain in research, which has stringent requirements for
data quality and completeness.
By far the most common expression of gestational weight gain in
research is total weight gain based on the difference between an initial
weight and a final weight taken in the last few weeks prior to delivery
(Method 1 in Bible 4-3~. Rarely is it corrected for the weight of the fetus,
placenta, and amniotic fluid in an effort to assess net weight gain or maternal
tissue change (Method 2~. Occasionally, total weight gain is corrected for
gestational length to yield a rate of gain (Method 3~. Increasingly, both
researchers and clinicians are expressing gestational weight gain relative
to either the observed or desirable prepregnancy weight (Method 4) or
some recommended weight gain (Method 5), in which case the reference
body weight (actual or desirable) should be expressed relative to maternal
stature.
All the methods of expressing gestational weight gain shown in Bible
4-3 have been used in research. A premium is placed on reliable weight
measurements, accurate estimates of gestational age, and valid reference
standards. Total weight gain based on recalled prepregnancy weight without
correction for length of gestation has severe limitations. This is particularly
true in the study of mechanisms by which weight gain and, more specifically,
changes in maternal tissue and nutritional status affect maternal and fetal
well-being. Measures of rate of weight gain, especially for specific phases
of gestation, provide insight into the timing of intrauterine insults and,
perhaps, into the mechanisms of fetal growth retardation and pregnancy
and postpartum complications. However, the data needed to compute
weight gain by stages of gestation are difficult to collect with adequate
quality in all but the best-controlled prospective studies.
Prepregnancy Weight Estimates
Recalled prepregnancy weights may be acceptable for research under
many circumstances (Stevens-Simon et al., 1986~; however, U.S. women of
reproductive age tend to report current weights approximately 1.4 kg (3
lb) lower than their actual weights. Moreover, U.S. women who perceive
themselves as overweight or who have less than a high school education
underestimate their weight to a greater extent than do those who believe
OCR for page 87
ASSESSMENT OF GESTATIONAL WEIGHT GAIN
87
their weights are normal or who have more formal education (Stewart,
1982~.
Unlike weight, height tends to be overreported by an average of 1.1
cm (0.42 in.), especially among short women with limited formal education
(Stewart, 1982~. Considering the negative bias in recalled weight and the
positive bias in self-reported height, BMIs based on self-reported rather
than measured prepregnancy weights and heights tend to be underestimated
to a greater degree than Is weight alone.
Weights determined at the first prenatal visit during the first trimester
of pregnancy have been used to estimate total weight gain and early-
gestation weight gain, but they do not necessarily reflect prepregnancy
weights. Although average weight gain in the first trimester is small rel-
ative to that in later periods of pregnancy, individual variation may be
considerable. Total gestational weight gains may be overestimated by self-
reporting or underestimated if based on late first trimester weight.
Gestational Duration Estimates
The use of reported LMP to estimate gestational duration has been
shown (Kramer et al., 1988) to result in substantial misclassification of
preterm and postterm (>42 weeks of gestation) births and thus, possi-
bly, incorrect diagnosis of intrauterine growth retardation. This error in
gestational age could cause an overestimate of the rate of weight gain in
women delivering at full-term but whose LMP dates suggest a preterm
birth. Kramer et al. suggest an even greater error in overestimating gesta-
tional age among women with postterm LMP dates. Women who deliver
their infants at full-term rather than postterm (as estimated from their
LMP) may have an underestimate of their true rate of gain when that rate
is calculated based on the LMP gestational age.
1b improve upon estimates of gestational length based on LMP dates,
several methods have been used during pregnancy (sonography, symphysis-
uterine fundal height) and at delivery (newborn maturity indicators). All
these methods are based on measurements that are dependent upon length
of gestation. In all cases, the estimates of gestational age were validated
against gestational duration based on LMP, which is itself an imperfect
measure. Moreover, all these techniques are based on measurements of
fetal development that are also influenced by intrauterine environmental
factors. Therefore, a 20-week-old fetus (whose age was estimated by
sonography) who is determined to be growth retarded to the size of a
16-week-old fetus will be given a gestational age of 16 weeks, and the
gestational weight gain of the mother may be charted at 16, not 20, weeks
from conception. If there is an association between gestational weight gain
and fetal growth, this relationship might be obscured by the fact that both
do.
OCR for page 88
g8
NUTRITIONAL STATUS AND WEIGHT GAIN
the dependent variable (fetal weight or birth weight for gestational age)
and the independent variable (weight gain) are biased in the same direction
by errors in estimating gestational age.
Determining Final Weight and Rates of Weight Gain
Rarely is the final maternal weight measured at admission to the
hospital just before delivery, but rather, it is measured at the last prenatal
visit. Moreover, weights taken at the hospital vary according to the status
of the membranes, i.e., whether or not they have ruptured, the content
of the bladder and bowel, and the amount of clothing worn during the
weighing, which is likely to have been done in a hurry.
An alternative to using total weight gain is to compute a rate of weight
gain up to the date of the last prenatal visit. Of course, any estimate of a
rate of weight gain based only on initial and final weights assumes a linear
pattern, which is not likely to be the case. Most evidence suggests that the
lowest rates of weight gain occur during the earliest and latest weeks of a
term pregnancy (see Chapter 5), when the frequency of the weighings may
be most variable. A nonlinear pattern of weight gain plus the irregular
time between weighings could result in more error in estimating the rate
of weight gain as opposed to total weight gain. For example, a woman
who gains a total of 11 kg (25 lb) between 12 and 37 weeks of gestation
would have gained weight at a linear rate of 0.5 kg/week (1 lb/week). But
if (as sometimes occurs) this initial weight is used as the prepregnancy
weight (i.e., weight at week zero of gestation) and the weight at the last
prenatal visit (e.g., at 37 weeks of gestation) is used as the delivery weight
(e.g., at 39 weeks of gestation), then the computed rate is 0.3 kg/week
(0.64 lb/week). More frequent weighings enable investigators to make
more precise estimates of weight gain patterns according to the trimester
of pregnancy, and would facilitate identification of the occurrence of fetal
or maternal health problems by period of gestation.
Including the Fetus in Total Weight Gain
Total gestational weight gain includes the products of conception as
well as maternal tissue (Method 1 in Table 4-3~. The definition may lead
to problems of misinterpretation of studies relating maternal weight gain
to birth weight. When the weight of the newborn is included in both
the dependent and independent variables, a statistical situation known as
part-whole correlation occurs. This is of considerable concern in research
on maternal factors relating to variation in fetal growth when causal mech-
anisms are being investigated (see Chapter 8~. In clinical settings, however,
where total weight gain is used as a screening tool to assess fetal well-being,
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ASSESSMENT OF GESTATIONAL WEIGHT GAIN
89
it is advantageous that maternal weight gain reflects fetal size, especially if
the screening takes place at a stage of pregnancy that allows intervention.
~ estimate maternal tissue mass more closely, researchers generally
subtract the infant's birth weight from the total weight gain. At times, this
procedure is refined by subtracting placental weight and an estimate of
amniotic fluid weight. Or, when the data are available, net weight gain may
be estimated by subtracting prepregnancy weight from postpartum weight
obtained after delivery.
Comparison of Gestaitonal Weight Gain with Reference Standards
The expression of maternal weight or weight gain relative to a reference
standard is especially useful if initial weights were taken after the first
trimester of pregnancy and total or cumulative weight gain could not be
calculated. One approach is to express weight at a given gestational age
relative to prepregnancy body weight or desirable body weight (Method 4
in Table 4-3~. This is based on the assumption that weight gain should be
proportional to either initial or desirable body weight. These two ways of
expressing the reference prepregnancy weight can yield very different results
at the extremes of the observed body weight distribution in a population.
An overweight woman who is 165-cm (65-in.) tall and whose prepregnancy
weight was 80 kg (176 lb) serves as an example. If she gains 15 kg (33 lb)
during a pregnancy, she achieves 119% of her actual prepregnancy weight
(95 kg/80 kg) but 161% of her "ideal" prepregnancy weight (i.e., 59 kg,
according to 1959 MLI tables). On the other hand, a woman of the same
height who weighs 45 kg (99 lb) and gains 15 kg (33 lb) during pregnancy
would achieve 133% of her observed prepregnancy weight and 102% of her
ideal weight.
One advantage to the use of ideal prepregnancy weight as a reference
weight is that it does not require knowledge of actual prepregnancy weight.
Also, any analysis that includes weight gain expressed as a percentage of
observed prepregnancy weight will be confounded by prepregnancy weight,
the very factor that is independently related to pregnancy outcome and that
may modify the effect of weight gain on the outcome.
Using ideal body weight as the reference weight from which total pre-
scribed weight gain is estimated, Gueri et al. (1982) presented a rationale
for recommending a total gestational weight gain equal to 20% of the
mother's ideal weight for height. They based their weight gain recommen-
dations on four assumptions: (1) the desirable prepregnancy weights for
height used are the average weights for U.S. women of different heights,
as obtained for women between 18 and 24 years of age in the NHANES I
survey, (2) the average total gestational weight gain should be 12 kg (26.4
lb), (3) the average increment of gestational weight gain should be 20% of
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NUTRITIONAL STATUS AND WEIGHT GAIN
the ideal prepregnancy weight, and (4) the average gestational weight gain
of 1 kg (2.2 lb) in the first trimester and the gain of 11 kg (24.2 lb) in the
next two trimesters follow a linear pattern.
As illustrated in Figure 4-7, the target maternal weight at any gesta-
tional age differs according to maternal height. A total gain of 12 kg (26.5
lb) equals 20% of the average weight of U.S. women of average height (163
cm, or 64 in.) in the reference population. Ibller women have a higher
desirable body weight and would be expected to gain proportionately more
than 12 kg. Similarly, shorter women would be expected to gain propor-
tionately less weight. The adjustments for weight gain by trimester made
by Gueri and colleagues were based on the assumption that regardless of
the total prescribed weight gain, one-twelfth (8.3%) of the total gain should
occur in the first trimester, and the remaining eleven-twelfths of the total
gain should be divided equally between the remaining two trimesters (i.e.,
45.8% of the total gain in each trimester).
The application of this recommendation to women at the extremes
of the weight-for-height spectrum poses a problem. Underweight women
would be expected not only to compensate for their entire deficit in prepreg-
nancy body weight but also to add the tissues of pregnancy during a 9-month
period. For a 160-cm (63-in.) tall woman whose prepregnancy weight was
47.5 kg (104 lb), this would amount to a gain of 23.5 kg (52 lb) 50%
of her prepregnancy weight-during the course of the pregnancy. The
subcommittee found no evidence that such a large weight gain would be
desirable for underweight women. Another serious problem with the table
of Gueri et al. occurs at the upper end of the range of prepregnancy weight
for height. Women at 120% of the reference weight would be advised to
gain no weight during pregnancy, and those over 120% of the reference
weight would be advised to lose weight during pregnancy.
Husaini et al. (1986) developed a weight gain grid for Indonesian
women based on the same principles used by Gueri and colleagues, but they
used observed weight gain among women who delivered infants weighing
more than 2,500 g (5.5 lb) at birth. Overweight was not a factor in
this poor, rural Javanese population, so their patterns of weight gain
for different heights appear to follow those presented by Gueri et al.
However, the achieved weights at any given height and at any given week
of gestation are considerably lower than those suggested by Gueri and
associates. Furthermore, the weight gain chart for Indonesian women does
not assume a proportionate weight gain; short women of any prepregnangy
weight would gain the same absolute amount of weight during gestation as
taller women of the same relative weight for height.
Another modification of this approach is to express weight gain as
a percentage of recommended gain, either over the entire pregnancy or
over specified periods (Method 5 in Table 4-3~. This requires the same
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ASSESSMENT OF GESTATIONAL WEIGHT GAIN
91
measurements as in Methods 1 and 3 but expresses the resulting gains
relative to a standard. This is similar to the way one expresses weight or
height in children relative to a postnatal growth standard. The advantage
of this method is that it allows weights collected at different frequencies
during different stages of pregnancy to be compared on a similar scale (as
percentages of reference weight gain) and thus removes the time depen-
dence of weight changes described above. This is also the preferred way
of expressing gestational weight gain when studying its effect on length
of gestation or risk of premature delivery. Since premature delivery will
shorten the time over which weight gain occurs, there is a need to ex-
press weight gain in a way that is independent of the length of gestation.
Many researchers choose to express weight gain as a rate. However, this
approach usually does not account for the nonlinear nature of weight gain
throughout gestation, which will result in a bias toward underestimating
the true rate of weight gain if the pregnancy is terminated early. This bias
could lead to an observed negative relationship between rate of weight gain
and risk of preterm delivery that may not actually exist. One solution to
this problem is to express weight gain as a percentage of a reference weight
for a given week of gestation. Of course, the method is valid only if the
correct standard is used and if the percentage deviation from the reference
curve is interpreted similarly at different weeks of gestation.
Given current knowledge of the use of this approach to evaluate early
postnatal growth, it is likely that a given percentage of reference maternal
weight for gestational age at 20 weeks of gestation implies a different
degree of risk than the same percentage of reference weight at 35 weeks of
gestation (Haas and Habicht, 1990~. At the very least, the method requires
knowing if the coefficient of variation (standard deviationJmean) of weight
gain in normal pregnancies with favorable outcomes is constant over all
gestational ages. If it is not constant, then a given percent deviation from a
mean weight at a given week of gestation will not have the same percentile
standing as it would at another week of gestation.
USES OF GESTATIONAL WEIGHT GAIN IN
NUTRITIONAL SURVEILLANCE
In recent years, a system of nutritional surveillance has been under
development in the United States. The major objective of such a system
is to provide accurate information to decision makers who are responsible
for policy, program development, and management in areas that affect
nutrition and public health (DHHS/USDA, 1986; Mason et al., 1984~. The
information derived from nutritional surveillance can also be of use in
clinical practice (Won" and Footbridge, 1984~. The Centers for Disease
Control have been charged with the coordination of nutrition surveillance
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92
NUTRITIONAL STATUS AND WEIGHT GAIN
activities, which include identifying data sources, standardizing data col-
lection instruments, training people in data collection, conducting data
analysis, and disseminating information to appropriate users. Two compo-
nents of nutritional surveillance related to the work of this subcommittee
are the Pregnancy Risk Assessment Monitoring System and the Pregnancy
Nutrition Surveillance System. The goal of these two systems is to con-
tribute to the reduction of pregnancy-related health risk factors and adverse
pregnancy outcomes in both the general population and the low-income
population of the United States.
In these surveillance systems, gestational weight gain is included as a
maternal measure associated with health risk and pregnancy outcome. The
extensive standardized data collection instrument developed for these two
systems includes questions on prepregnancy weight and height as well as
total gestational weight gain and length of gestation (from LMP). There
appears to be no information on patterns of weight gain or a specific
identification code for the sources of data on weight and height (measured
versus recalled). Since these systems are still in the development stage, it
has not been possible to evaluate how these data are to be analyzed and
whether some of the concerns raised elsewhere in this chapter have been
addressed.
The analytical depth and sophistication often required of scientific
research cannot be applied to nutritional surveillance data. Reports must
be generated in a timely fashion if they are to be of use to decision makers,
and a sophisticated statistical analysis may not be understood by many of
the potential users. However, most of the concerns regarding data quality
and interpretation raised under research applications also apply to use of
gestational weight gain data in nutritional surveillance.
Operation of the two relevant surveillance systems may produce ben-
efits in addition to those previously stated. Because of increased efforts
to standardize data collection methods among the many state programs,
there is likely to be an increase in the amount and quality of data available
on gestational weight gain, its antecedents, and its consequences, so that
important research can progress. Also, this standardization process may
extend to clinical uses of gestational weight gain charts such as those found
in individual state WIC programs.
SUMMARY
Clinical practice, research, and surveillance activities benefit from
the implementation of proper measurement techniques. Interpretation of
results from studies of gestational weight gain requires careful attention to
definitions of gestational weight gain and gestational duration.
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ASSESSMENT OF GESTATIONAL WEIGHT GAIN
CLINICAL IMPLICATIONS
93
· Prepregnancy weight is an important factor. Objective data, such as
those obtained from a medical record, are preferred. Information provided
by the patient should be evaluated for its accuracy.
· Height without shoes should be determined at the first prenatal
visit, preferably with a wall stadiometer whose accuracy has been verified.
· Gestational age should be estimated from the date of onset of the
woman's last menstruation, preferably supplemented by estimates based on
the obstetric clinical examination and perhaps by early ultrasound.
· A weight-for-height category is derived from the patient's height
and prepregnancy weight. The resulting BMI can be compared to the
recommended classification in Appendix C.
· This comparison will provide a foundation for specific nutrition
counseling and the creation of a plan for overall and incremental weight
gain.
The patient's weight should be obtained at the beginning of each
prenatal visit. Consistently, the patient should be weighed without her
purse and outdoor clothing. If weight cannot be obtained routinely without
shoes, a marked variation in the type of footwear should be noted.
.
The immediate recording of measurements directly on an appro-
priate form is encouraged.
· Recording data on a chart that depicts weight gain by gestational
age provides a visual impression of sequential changes. Abrupt or inconsis-
tent changes should be scrutinized to determine whether they reflect errors
or accurate data.
· Members of the clinical staff should be trained in proper measure-
ment techniques. Their performance should be monitored periodically.
· Equipment for taking measurements should be calibrated periodi-
cally.
· Only the patient's approximate weight is obtained at the time of
measurement. An exact measurement is not possible because of clothing,
contents of bladder and bowel, time of day, time of last meal, and other
such factors. Therefore, interpretation of weight change should focus on
sequential data or trends.
The patient's weight should be compared with the target weights
and rates of gains depicted in Chapter 1, Table 1-1, and Appendix B.
Investigation to determine the cause of inappropriate patterns will permit
specific remedial action to be taken.
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Representative terms from entire chapter:
prepregnancy weight
