Objective: The purpose of this study was to examine body mass index and pregnancy weight gain as risk factors for primary cesarean delivery in nulliparous women in a middle-class private practice.
Study design: Primiparous women who delivered in a private practice setting between February 1993 and July 13, 2001, were included. CIs along with Z statistics for paired count data were used to assess the statistical significance and relative importance of the relationships of body mass index and maternal weight gain to cesarean delivery. The effect of body mass index was examined as related to known confounders such as gestational age, birth weight, maternal age, and maternal height.
Results: The overall cesarean delivery rate for primiparous women was 21.76%. Risk of cesarean delivery increased consistently and significantly (P <.0001) with increasing body mass index. This effect was primarily mediated through an increase in cesarean delivery carried out for cephalopelvic disproportion/failure to progress. In our practice, the primiparous woman whose body mass index is >30 kg/m(2) is six times more likely to have a cesarean delivery for the diagnosis of cephalopelvic disproportion/failure to progress than the primiparous woman whose body mass index is <20 kg/m(2). This differential in cesarean delivery rate persisted when controlled for birth weight and gestational age and continues to persist when maternal age and height are also controlled. Excessive pregnancy weight gain exerted a statistically significant effect on cesarean delivery rate. This increase was primarily related to cephalopelvic disproportion/failure to progress among the nonobese women.
Conclusion: Maternal body mass index is related strongly to the ability of primiparous women to be delivered vaginally without great difficulty. In fact, lean patients are excellent labor performers, particularly in contrast with obese patients. The relationship of increased body mass index to increased cesarean delivery is due to an increased rate of cephalopelvic disproportion/failure to progress. Excessive pregnancy weight gain is associated with a doubling of cephalopelvic disproportion/failure to progress rate in nonobese patients.