Background: The prevalence of obesity substantially increases in pregnant women. Maternal obesity is associated with adverse maternal and neonatal outcomes. The increased risk for cesarean section present in obese women has been related to potential impaired uterine contractility. The mechanism that underlies this theory is not clear. In vitro studies have shown that leptin, produced by adipose tissue and the placenta, exerts an inhibitory effect on myometrial contractility. The aim of this thesis was to evaluate the labor process in relation to maternal body mass index (BMI) and the clinical role of leptin in this process.

Material and Methods: Studies I-IV are cohort studies. The first two studies analyze the association between labor duration and maternal BMI based on data from the Perinatal Revision South register and the Swedish Pregnancy Register. Study I included 63,829 nulliparous women with a spontaneous onset of labor between 1995 and 2009. Study II included 15,259 nulliparous women with induced labor between 2014 and 2017. In study III, the maternal leptin levels during and after pregnancy were analyzed in 343 obese women with respect to their obesity class (I-III) and degree of gestational weight gain (GWG). In study IV, the association between the maternal leptin levels measured in active labor and duration of the active phase of labor was analyzed in 914 women.

Results: The duration of spontaneous labor significantly increased with an increasing maternal BMI; however, the duration of the pushing phase was inversely related to BMI. Time in induced labor increased with maternal BMI; however, the differences between the BMI categories were more pronounced in the latent phase than the active phase. Leptin levels were higher in women with obesity class III than women with class I during and after pregnancy. The degree of GWG in obese women was not associated with maternal leptin. No significant association between maternal leptin and the duration of the active phase of labor was identified in the adjusted analyses.

Conclusions: Nulliparous obese women have a higher risk for a prolonged duration of spontaneous and induced labor. This is important to consider prior to diagnosing labor arrest that results in a cesarean delivery. As maternal leptin levels are increased with the degree of obesity during pregnancy, future research on the association of high maternal leptin levels and the duration of labor is warranted.