Of 2,012 mothers, 61% were of normal weight entering pregnancy (BMI 18.5–24.9 kg/m2), 23% were overweight (BMI 25.0–29.9 kg/m2), and 16% were obese (BMI ≥30 kg/m2) (). Within each BMI category, the rate of gestational weight gain was approximately normally distributed. The mean rate of total gestational weight gain was 0.39 kg/week (standard deviation, 0.14; range, −0.19 to 1.03) overall, 0.41 kg/week (standard deviation, 0.12; range, −0.05 to 1.03) for normal-weight women, 0.40 kg/week (standard deviation, 0.15; range, −0.03 to 0.88) for overweight women, and 0.31 kg/week (standard deviation, 0.19, range, −0.19 to 0.93) for obese women. Only one woman in each of the normal-weight and overweight BMI groups lost weight during pregnancy, but 12 (1.2%) obese women lost weight. The rate of total gestational weight gain to delivery was 0.35 kg/week among those who delivered preterm. The rate of gestational weight gain to week 32 of gestation was 0.35 kg/week among those who delivered preterm and 0.38 kg/week among those who delivered at term.
Prevalence of 5 Maternal and Child Health Outcomes According to Characteristics of 2,012 Massachusetts Mothers Enrolled in Project Viva, 1999–2002
The prevalence of SGA was 6% and the prevalence of LGA was 14%; thus, babies in this cohort were somewhat heavier compared with US population norms of 10% for both outcomes. The prevalence of preterm delivery was 7%, of substantial postpartum weight retention was 16%, and of child obesity was 10%. In , we present the prevalence of these 5 outcomes according to selected maternal characteristics. Gestational weight gain was not associated with risk of cesarean section (data not shown).
Among all women, gestational weight gain was directly associated with risk of substantial postpartum weight retention and child obesity, although estimates for child obesity were less strong and confidence intervals crossed 1.0 among normal-weight and obese women (). Gestational weight gain was also directly associated with risk of LGA among normal-weight and overweight women, whereas it was inversely associated with risk of SGA and preterm delivery among normal-weight women. Among overweight women, associations of higher gestational weight gain with lower risk of SGA and preterm birth were not statistically significant. Among obese women, gestational weight gain was not associated with risk of LGA, SGA, or preterm birth.
Associations of Total Gestational Weight Gain With 5 Adverse Outcomes Among 2,012 Massachusetts Mother-Child Pairs Enrolled in Project Viva, 1999–2002a
In , we display the predicted prevalence of each of the 5 adverse outcomes according to the total gestational rate of gain, stratified by maternal prepregnancy BMI. The left and right limits of the lines in each panel represent the lowest and highest observed values, respectively, of rate of gestational gain within that BMI stratum.
Figure 1. Predicted probabilities of small-for-gestational-age birth, large-for-gestational-age birth, preterm delivery, maternal substantial postpartum weight retention, and child obesity according to rate of gestational weight gain during pregnancy among women (more ...)
The lowest predicted prevalence of all 5 adverse outcomes occurred with a gestational gain of 0.28 kg/week for normal-weight women, a loss of 0.03 kg/week for overweight women, and a loss of 0.19 kg/week for obese women ( and ). For overweight and obese women, these values represent the lowest weight change observed. Over 40 weeks of gestation, these rates would correspond to a gain of 11.2 kg for normal-weight women, a loss of 1.2 kg for overweight women, and a loss of 7.6 kg for obese women.
Table 3. Rates of Total Gestational Weight Gain (kg/week) Associated With the Lowest Predicted Prevalence of 5 Adverse Outcomes,a in Models Unadjusted, Adjusted, and With Different Outcome Weights Applied, Among 2,012 Massachusetts Mother-Child Pairs Enrolled (more ...)
Figure 2. Average predicted probability of 5 adverse outcomes (small-for-gestational-age birth, large-for-gestational-age birth, preterm delivery, maternal substantial postpartum weight retention, and child obesity) according to maternal prepregnancy body mass (more ...)
Estimates for normal-weight women varied somewhat according to the value of maternal BMI we used for predicted models. If, instead of the mean BMI (22.1 kg/m2), we used the lowest observed value (18.5 kg/m2), the lowest predicted outcome prevalence occurred at a gain of 0.36 kg/week, whereas if we used a BMI of 24.9 kg/m2, the lowest predicted outcome prevalence occurred at a gain of 0.21 kg/week. Estimates for overweight and obese women did not change according to the value of maternal BMI used in prediction models.
Estimates for the gain associated with the lowest risk were similar when we assumed that all women underreported their prepregnancy weight by 1 kg (0.27 for normal-weight, −0.07 for overweight, and −0.21 kg/week for obese women) or by 10% (0.31 for normal-weight, −0.03 for overweight, and −0.17 kg/week for obese women) or when we categorized maternal BMI according to the cutpoints used in the 1990 Institute of Medicine report (0.27 for normal-weight, 0.08 for overweight, and –0.19 kg/week for obese women). Results were also similar when we restricted our analysis to the 985 mother-child pairs with information on all 5 outcomes or when we modeled gestational weight gain in categories rather than as a continuous exposure (data not shown). When we included maternal race/ethnicity, age, parity, and smoking in multivariate models, estimates varied minimally for normal-weight and overweight women and did not change for obese women ().
For the above analyses, we assumed that each of the 5 outcomes was equally undesirable. We next explored the effect of weighting the outcomes. In our convenience survey, 5 local experts ranked preterm birth as the worst outcome, with an average weight of 6. Three ranked child obesity as the worst outcome, with an average weight of 2; 3 ranked substantial postpartum weight retention as the worst, with an average weight of 3; and 1 ranked SGA as the worst, with a weight of 6. None ranked LGA as the worst outcome. Weighting preterm birth 6-fold worse or SGA 5-fold worse than the other outcomes shifted estimates of lowest-risk gestational weight gain higher, whereas weighting child obesity 2-fold worse or maternal substantial postpartum weight retention 3-fold worse than the others shifted estimates somewhat lower (). Estimates of lowest-risk gain for obese women did not shift from the lowest observed value in any of the models (). When all of these weights were included simultaneously, lowest-risk gains were above the range currently recommended by the Institute of Medicine for normal-weight women (0.42 kg/week) but remained below current recommendations for overweight and obese women. In all models, the lowest-risk weight change among obese women was the lowest observed value, that is, weight loss during pregnancy.