Background: To investigate whether maternal anthropometric factors interact with one another or with other risk factors, thus modifying the risk of spontaneous preterm delivery.
Methods: We carried out a case-control study of 230 spontaneous preterm births with intact membranes between 24 and 35 weeks gestation and 460 control term births. All the patients had prenatal care at the same institution. Logistic regression analysis was used to test for possible interactions adjusting for potential confounders.
Results: A pre-pregnancy body mass index < or = 19.5 Kg/m2 (odds ratio (OR) = 1.68, 95% confidence interval (CI) = 1.20-2.38) and a rate of weight gain < or = 0.37 Kg/week during the second and third trimesters (OR = 2.4, 95% CI = 1.69-3.42) were associated with an increased risk of spontaneous preterm delivery. The risk of spontaneous preterm delivery associated with a low second/third trimester weight gain was greater among patients with a body mass index < or = 19.5 (OR = 5.63, 95% CI = 2.35-13.8) compared to those with a body mass index > 19.5 (OR = 2.45, 95% CI = 1.60-3.75, adjusted p value for interaction = 0.05). The risk of spontaneous premature delivery associated with a maternal pre-pregnancy weight < or = 48 Kg was higher among smokers (OR = 5.81, 95% CI = 1.60-22.9) than among non-smokers (OR = 2.4, 95% CI = 1.53-3.74, adjusted p value for interaction = 0.05).
Conclusions: The risk of spontaneous preterm delivery associated with a low pre-pregnancy body mass index is greater among patients with low rate of gestational weight gain during the second and third trimesters compared to those with a higher rate. The results of this study support the recommendation for increased rates of weight gain to patients with low body mass index compared to those with a higher body mass index.