Body Mass Index and the Spontaneous Onset of Parturition.

OBJECTIVE: To investigate the relationship between body mass index (BMI) and the onset of parturition throughout gestation.

METHODS: This was a secondary analysis of the Maternal-Fetal Medicine Units Network Preterm Prediction Study. Time-to-spontaneous-birth-event (ie, "survival") methods were used to study the association of BMI with the timing of spontaneous onset of labor throughout gestation with indicated births censored at delivery. A Kaplan-Meier estimate of the probability of spontaneous labor was compared with a log rank test across five categories of BMI (kg/m): underweight (less than 18.5), normal weight (18.5-24.99), preobese (25-29.99), obese I (30-34.99), and obese II+ (35 or greater). A proportional hazards model was estimated to compare time to spontaneous onset of labor adjusted for multiple variables known to be associated with the onset of labor.

RESULTS: Normal-weight women (n=1,054) had a median delivery gestational age of 39 3/7 weeks. Obese II+ women (n=178) had a median delivery gestational age 5 days later than normal-weight women (P<.001). Delivery gestational age of preobese (n=866) and obese I (n=548) women was not significantly different from normal-weight women. Underweight women (n=41) had a median delivery gestational age 5 days earlier than normal-weight women (P<.001). Compared with women with normal BMIs, obese II+ women were significantly less likely and underweight women significantly more likely to enter spontaneous labor at all gestational ages. In the multivariable model, BMI was significantly associated with spontaneous onset of labor throughout pregnancy (BMI [five-unit] adjusted hazard ratio 0.874, 0.829-0.921).

CONCLUSION: Body mass index is significantly associated with the likelihood of the spontaneous onset of labor at all gestational ages with gestational age at the time of delivery and BMI being inversely related. This novel observation unifies previous reports focusing on the association of overweight and underweight BMIs and preterm and postterm birth and may inform discussions surrounding elective induction of labor at term.