Results
The average age of the study sample was 24.1 years (standard deviation (SD) = 6.0 years) ( Table 1 ). The racial distribution consisted of a slightly higher proportion of Blacks (48.5%) than Whites (44.4%). The majority reported to be married (76.2%) and having less than high school education (56.9%). The majority of births were to multiparous women (71.2%) who delivered full term (86.1%) infants. However, nearly 8% had spontaneous preterm birth with PROM, 5% delivered a spontaneous preterm birth without PROM, and 1.2% of preterm births were indicated. Over two-thirds of the study population had normal pre-pregnancy weight (68.9%); and 15% and 6% were overweight and obese, respectively.
Figure 1 displays the prevalence rate of the preterm birth sub-types by pre-pregnancy BMI and weight gained during pregnancy. Among underweight and normal weight pregnant women, the rates of all sub-types of preterm births decreased with increased gestational weight. On the other hand, in overweight women, the rate of preterm birth with PROM showed a reversed J-shape with increasing weight gain. Similarly, among obese women, the rates of all sub-types of preterm births were highest among those who gained the lowest (<7.0 kg) and highest weight categories for spontaneous preterm birth with and without PROM.
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Figure 1.
Prevalence of preterm birth by pre-pregnancy BMI and pregnancy weight gain.
Table 2 shows the unadjusted association between obesity, gestational weight gain and preterm birth. Gaining less weight (< 7.0 kg) was significantly associated with increased odds of spontaneous preterm birth with and without PROM in underweight women. However, gaining less weight (< 7.0 kg) was associated with decreased odds of all types of preterm birth in overweight women and spontaneous preterm birth with and without PROM in obese women. Gaining 7.0–9.4 kg was statistically associated with spontaneous preterm birth without PROM in obese women compared to normal weight women. Additionally, gaining 9.5–12.7 kg was statistically associated with increased odds of spontaneous preterm birth with and without PROM and indicated preterm birth in underweight and obese women, respectively. The odds of spontaneous preterm birth without PROM were higher among obese women who gained >12.7 kg compared to normal weight women.
Stratified analysis by race showed underweight women who gained less weight (<7.0 kg) had increased odds of spontaneous preterm birth with and without PROM, regardless of their race ( Table 3 ). Conversely, gaining >12.7 kg was associated with decreased odds of spontaneous preterm birth with PROM in underweight women compared to normal weight women. Overweight and obese women who gained less weight (<7.0 kg) had decreased odds of spontaneous preterm birth with and without PROM regardless of their race. While no difference was observed in Whites, overweight and obese Black women who gained 7.0–9.4 kg had lower odds of spontaneous preterm birth without PROM compared to normal weight women. Similarly, gaining 9.5–12.7 kg in Black obese women was associated with lower odds of spontaneous preterm birth without PROM compared to normal weight women.
Underweight women who gained less than 7.0 kg or between 9.5 and 12.7 kg during pregnancy had significantly increased odds of spontaneous preterm birth with PROM (OR = 1.70, 95% CI = 1.38–2.09) and without PROM (OR = 1.74, 95% CI = 1.36–2.22) compared to normal weight women. Similarly, the odds of spontaneous preterm birth with PROM (OR = 1.43, 95% CI = 1.15–1.78, and without PROM (OR = 1.57, 95% CI = 1.22–2.02) was higher among underweight women gaining 9.5–12.7 kg.
Among overweight and obese women, gaining less than 7.0 kg during pregnancy was statistically significantly associated with decreased odds of spontaneous preterm birth with and without PROM, compared to normal weight women. Further, gaining less weight (<7.0 kg) among overweight women was significantly associated with indicated (OR = 0.68, 95% CI = 0.41–0.99) compared to normal weight women.
Overweight and obese women who gained 7.0 to 9.4 kg during pregnancy had decreased odds of spontaneous preterm birth with or without PROM compared to normal weight women. Among overweight women, this weight gain was associated with nearly 30% decreased odds of spontaneous preterm birth with PROM (OR = 0.72, 95% CI-0.56–0.92) and over 40% decreased risk of spontaneous preterm birth without PROM (OR = 0.58, 95% CI-0.42–0.80). Obese women who gained 7.0 to 9.4 kg were also significantly less likely to have a spontaneous preterm birth with PROM (OR = 0.66, 95% CI = 0.44–0.99) and spontaneous preterm birth without PROM (OR = 0.28, 95% CI = 0.14–0.58) than normal weight women Further, obese women who gained 9.5–12.7 kg during pregnancy had lower odds of spontaneous preterm birth without PROM compared to normal weight women (OR = 0.45, 95% CI = 0.23–0.89). Additionally, obese women who gained 9.5–12.7 kg were 2.7 times more likely to have an indicated preterm birth compared to normal weight women (OR = 2.66, 95% CI = 1.35–5.22).