Increasing maternal body mass index and characteristics of the second stage of labor.

Objective: To evaluate the length of the second stage of labor in relation to increasing maternal prepregnancy body mass index (BMI) among nulliparous parturient women, and to determine whether route of delivery differs among obese, overweight, and normal-weight women reaching the second stage of labor.

Methods: We performed a secondary analysis of a multicenter trial of fetal pulse oximetry conducted among 5,341 nulliparous women who were induced or labored spontaneously at 36 weeks or more of gestation. Normal weight was defined as BMI of 18.5-24.9 kg/m, overweight was a BMI of 25.0-29.9 kg/m, and obese was a BMI of 30 or higher.

Results: Of the 5,341 women, 97% had prepregnancy BMI recorded. Of these, 3,739 had BMIs of 18.5 or higher and reached the second stage of labor. Increasing maternal BMI was not associated with second stage duration: normal weight, 1.1 hour; overweight, 1.1 hour; and obese, 1.0 hours (P=.13). Among women who reached the second stage, as BMI increased, so did the likelihood that the woman had undergone induction of labor. Even so, the lack of association between second-stage duration and BMI did not vary by method of labor onset (P=.84). The rate of cesarean delivery in the second stage did not differ by increasing BMI (normal weight 7.1%, overweight 9.6%, obese 6.9%, P=.17).

Conclusion: Among nulliparous women who reach the second stage of labor, increasing maternal BMI is not associated with a longer second stage or an increased risk of cesarean delivery.

Level Of Evidence: II.