ity, smoking status, or other maternal attributes has been sparse, and the few studies summarized in the AHRQ review inconsistent. In addition to prepregnancy BMI, the only other factor that appears to impact the association between GWG and birth weight for gestational age is time during pregnancy that GWG occurs, with modest support for a stronger effect of GWG that occurs during the first or second trimester than during the third trimester GWG (Viswanathan et al., 2008).
Preterm birth (< 37 weeks’ completed gestation) is a critical indicator of developmental maturity, with the risk of death and morbidity a direct function of the degree of prematurity. Specifically, births occurring at the margins, that is during 33-36 weeks’ gestation, are at modestly increased risk of health problems; births that occur < 33 weeks’ gestation are rarer events but at much greater risk. Morbidity risks associated with preterm birth include acute respiratory, central nervous system, and gastrointestinal disorders, long-term deficits in neurobehavioral development (IOM, 2007), and possibly adverse cardiometabolic outcomes (Hofman et al., 2004; Hovi et al., 2007). Although an early delivery may be the only alternative to intrauterine death in some instances, regardless of whether it is caused by natural processes or induced by clinical intervention (an increasingly common “cause” of preterm birth), the high and growing frequency of preterm birth in the United States makes this a critical endpoint to consider in relation to GWG.
At the time of the IOM (1990) report, the volume and quality of literature on preterm birth was quite limited. Several studies suggested that low GWG was associated with increased risk of preterm birth, but much of that may have resulted from the simple error of failing to recognize that the shortened period of pregnancy (i.e., preterm birth) limits the duration of time over which weight can be gained. Comparing total GWG between preterm and term births is meaningless since preterm birth, by definition, involves a shorter period of gestation, thereby truncating the opportunity for weight gain compared to term births.
Data generated on behalf of this committee (information contributed to the committee in consultation with: Herring [see Appendix G, Part II] and Stein [see Appendix G, Part III]) provided some of the first information on GWG and preterm birth to consider prepregnancy BMI, which is predictive of both preterm birth (higher risk with lower BMI) and GWG (higher GWG with lower BMI). The results of that effort suggested a modest U-shaped relationship between rate of net weight gain (the only proper measure to compare pregnancies of varying duration) and risk of preterm birth.
The AHRQ review (Viswanathan et al., 2008) included 12 studies on the relationship between rate of GWG and preterm birth. The studies show