摘要：Objective: The purpose of this study was to evaluate the r.l.tionship between prepregnancy maternal body mass index and spontaneous preterm birth and indicated preterm birth. Study design: This was a secondary analysis of the Maternal- Fetal Medicine Units Network, Preterm Prediction study. Patients were classified into categories that were based on their body mass index. rates of indicated and spontaneous preterm birth were compared. results: Five hundred ninety- seven (20.5% ) of 2910 women were obese. Obese women had fewer spontaneous preterm births at < 37 weeks of gestation (6.2% vs 11.2% ; P <. 001) and at < 34 weeks of gestation (1.5% vs 3.5% ; P =. 012). Women with a body mass index of < 19 kg/m2 had 16.6% spontaneous preterm birth, with a body mass index of 19 to 24.9 kg/m2 had 11.3% spontaneous preterm birth, with a body mass index of 25 to 29.9 kg/m2 had 8.1% spontaneous preterm birth, with a body mass index of 30 to 34.9 kg/m2 had 7.1% spontaneous preterm birth, and with a body mass index of ≥ 35 kg/m2 had 5.2% spontaneous eterm birth (P <. 0001). Indicated delivery was responsible for an increasing proportion of preterm births with increasing body mass index (P =. 001). Obese women had lower rates of cervical length < 25 mm (5% vs 8% pr.; P =. 012). Multivariable regression analysis confirmed a lower rate of spontaneous preterm birth in obese gravid women (odds ratio, 0.57; 95% CI, 0.39- 0.83; P =. 003). Conclusion: Obesity before pregnancy is associated with a lower rate of spontaneous preterm birth.