Objective: The perception that obese women have longer labors and a higher frequency of operative delivery because they are "poor pushers" persists despite the absence of objective study. We tested the hypothesis that obese women generate inadequate intrauterine pressure during the second stage of labor.
Methods: Intrauterine pressure was prospectively measured in 71 women during the second stage of labor. Obesity was defined as a body mass index (BMI) greater than 29 (n = 17). A BMI below 25 was normal (n = 40). Women with a BMI between 26 and 29 (n = 14) were considered overweight. All women labored with epidural analgesia and were alert and responsive throughout the study. After recording the baseline contractility, a standardized Valsalva maneuver was performed during contractions. The area under the intrauterine pressure curve (integral) was used as an estimate of uterine contractility.
Results: All women delivered vaginally. There were no significant differences in baseline uterine contractility among obese, overweight, and normal women either before (obese 1,787 mm Hg/s; 95% confidence interval [CI] 1,164, 2,742 versus normal 1,569 mm Hg/s; 95% CI 718, 2,371 versus overweight 1,770 mm Hg/s; 95% CI 1,305, 2,835; P =.223) or during Valsalva maneuver (obese 2,831 mm Hg/s; 95% CI 1,771, 4,599 versus normal 2,637 mm Hg/s; 95% CI 1,240, 4,390 versus overweight 2,813 mm Hg/s; 95% CI 1,209, 4,982; P =.742). A BMI greater than 25 was associated with a higher frequency of oxytocin augmentation (P =.037). Univariate analysis revealed a relationship between labor duration and BMI (r = 0.299, P =.018). Obese women labored longer during the active phase (one-way analysis of variance, P =.02), but second-stage duration was similar among groups (one-way analysis of variance P =.44). Obesity did not increase the incidence of perineal lacerations (P =.82) or frequency of operative delivery (relative risk obese versus nonobese = 0.212; 95% CI 0.04, 1.05).
Conclusion: Obese women produce second-stage intrauterine pressures equivalent to women with a normal BMI, although they may require oxytocin augmentation more often.
Level of evidence: II-2