Oxytocin and Oxytocinase in the Obese and Nonobese Parturients during Induction and Augmentation of Labor

Objective  To investigate differences in oxytocin (OXT) biodistribution between nonobese and obese parturients during labor. Study Design  Patients with body mass index (BMI) of either ≥ 18 ≤ 24.9 kg/m ² ("nonobese") or ≥ 30 kg/m ² ("obese") undergoing elective induction of labor were included ( N  = 25 each). Blood samples were collected at baseline (T ₀ ), and 20 minutes after maximal OXT augmentation or adequate uterine contractions (T ₁ ) for OXT and oxytocinase assays. A mixed-model repeated-measures analysis of variance was used to test for group versus time interaction and analysis of covariance to detect a difference in OXT level at T ₁ . Data presented as mean ± standard deviation or median (interquartile range), with p  < 0.05 considered significant. Results  The mean BMIs (kg/m ² ) were 22.1 ± 1.6 and 35.9 ± 5.1 in the nonobese and obese groups, respectively. No differences were observed in either the duration of OXT infusion, total dose of OXT, or plasma OXT (pg/mL) either at T ₀ or T ₁ . However, plasma oxytocinase (ng/mL) was significantly lower at T ₀ (1.41 [0.67, 3.51] vs. 0.40 [0.29, 1.12]; p  = 0.03) in the obese group. Conclusion  We provide preliminary evidence that the disposition of OXT may not be different between obese and nonobese women during labor.