Pre-Pregnancy Body Mass Index, Gestational Weight Gain, and Other Maternal Characteristics in Relation to Infant Birth Weight

Ihunnaya O. Frederick; Michelle A. Williams; Anne E. Sales; Diane P. Martin; Marcia Killien

Disclosures

Matern Child Health J. 2008;12(5):557-567. 

In This Article

Results

Study participants were predominantly white (68.4%) and 90.2% were educated beyond high school level. Approximately 8.5% were unmarried and 5.9% smoked during pregnancy. On the average, maternal age was 32.4 ± 5 years, pre-pregnancy BMI was 23.6 ± 5 kg/m2, total gestational weight gain was 16.3 ± 6 kg, and gestational age at delivery was 37.6 ± 3 weeks, (Mean ± SD) ( Table 1 ). Among lean women, 23.7% were below the IOM weight gain guideline of 12.5-18.0 kg, 45.0% were within guidelines, and 31.3% gained weight above the IOM guideline. Among women of average pre-pregnancy BMI, 13.1% were below their IOM weight gain guideline of 11.5-16.0 kg, 33.6% were within recommendation, and 53.3% gained weight above the IOM guideline. Approximately 7.3% of overweight women were below the IOM guideline of 7.0-11.5 kg, 17.0% were within recommendation, and 75.7% gained weight above guideline. Among obese women, 17.3% gained weight below 7.0 kg, 20.8% gained weight between 7.0-11.5 kg, while 61.8% gained weight above 11.5 kg ( Table 2 ).

When the nonlinear relationship between infant birth weight and pre-pregnancy BMI, previously suggested in bivariate analysis, was further assessed, the regression model that included the quadratic term of pre-pregnancy BMI (pre-pregnancy BMI squared) was a better fit for the data, compared to the simple linear model. This model accounted for 27.3% of the variation in infant birth weight (adjusted R2 = 0.273) ( Table 3 ). Both terms were statistically significantly associated with birth weight in the final model (pre-pregnancy BMI β = 44.67, P = 0.001; pre-pregnancy BMI2 β = −0.51, P = 0.029). Other covariates that were significantly associated with birth weight were: preterm delivery (β = −965.4, P < 0.001), nulliparity (β = −48.57, P = 0.015), female gender (β = −168.74, P < 0.001), and preeclampsia during the index pregnancy (β = −136.05, P = 0.025). Maternal race/ethnicity, educational attainment, marital and smoking status, and gestational diabetes during the index pregnancy were not associated with infant birth weight.

Lean women (pre-pregnancy BMI < 19.8 kg/m2) were at a 51% higher risk of delivering a LBW infant as compared with the referent group (pre-pregnancy BMI 19.8-20.6 kg/m2) (adjusted RR = 1.51; 95% CI 1.02-2.25) ( Table 4 ). When IOM guidelines were used to evaluate the risk of LBW and macrosomia, women who gained weight below guidelines experienced a 51% reduction in the risk of delivering a macrosomic infant compared to those whose gestational weight gains were within guidelines (adjusted RR = 0.49, CI = 0.31-0.78). The risk of LBW was not significantly associated with categories of IOM guidelines. Women who were above the IOM guideline experienced a 76% increased risk of delivering macrosomic infants compared with women below IOM guidelines (adjusted RR = 1.76, 95% CI 1.40-2.22). When median gestational weight gain (15.9 kg) was considered, even after adjusting for pre-pregnancy BMI, we noted that women who gained less than 15.9 kg during the index pregnancy experienced a 2.04-fold increased risk of delivering a LBW infant as compared with women who gained at least 15.9 kg (adjusted RR = 2.04, 95% CI 1.34-3.11). Lean women had a 50% reduced risk of delivering macrosomic infants as compared with the referent group (adjusted RR = 0.50; 95% CI 0.35-0.71). Notably, obese women had a 1.65-fold increased risk of delivering a macrosomic newborn as compared with the referent group (adjusted RR = 1.65; 95% CI 1.29-2.11). After adjusting for potential confounders including maternal pre-pregnancy BMI, race/ethnicity and complications during pregnancy, we noted that gestational weight gain below median (<15.9 kg) was associated with a 51% reduced risk of delivering macrosomic infants (adjusted RR = 0.49; 95% CI 0.40-0.60).

The interaction term between pre-pregnancy BMI and gestational weight gain was not significant in relation to infant birth weight (P = 0.645). It was also not significant in relation to LBW (P = 0.407) but was significant when macrosomia was considered (P < 0.011), and after adjusting for confounders (data not shown.). Compared to women in the first referent group, lean women whose weight gain was below median (<15.9 kg) experienced a 2.97-fold increased risk of delivering LBW infants (adjusted RR = 2.97, 95% CI 1.63-5.43) after adjusting for confounders. Overweight women who gained weight below the median experienced a two-fold increased risk of LBW (adjusted RR = 2.01, CI = 0.99-4.11) as compared with the referent group, although this association did not reach statistical significance ( Table 5 ). In addition, we noted a reduced risk of macrosomia (adjusted RR = 0.20, CI = 0.10-0.38) among lean women with less than median gestational weight gain, after adjustment for confounding. When women of average pre-pregnancy BMI with ≤median gestational weight gain (19.8-26.0 kg/m2 and ≤15.9 kg) were the second referent group, the risk of macrosomia increased among overweight and obese women regardless of pre-pregnancy BMI. The risk of LBW decreased among obese women with less than median gestational weight gain (adjusted RR = 0.60, CI = 0.36-0.99) ( Table 5 ).

Using ordered logistic regression procedures, we noted consistent inverse associations between predicted probabilities of LBW within each pre-pregnancy BMI category. The predicted probabilities of LBW decreased monotonically across increasing categories of pre-pregnancy BMI, from lean (PR = 0.020, 95% CI 0.012-0.027) to average (PR = 0.014, 95% CI 0.009-0.020), to overweight (PR = 0.011, 95% CI 0.006-0.014), to obese women (PR = 0.008, 95% CI 0.004-0.011) (data not shown). Conversely, the predicted probabilities of macrosomia increased monotonically across increasing categories of pre-pregnancy BMI. Compared to women who gained weight above median, those who gained ≤15.9 kg had lower predicted probability of macrosomia (data not shown).

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