Rates of cesarean section have increased in the past 2 to 3 decades in the United Kingdom and elsewhere. Apart from immediate maternal and infant complications, there appears to be an increased risk of placenta previa, placental abruption, and uterine rupture during labor in subsequent pregnancies. Recent studies suggest an increased risk of stillbirth in pregnancies following cesarean delivery. The investigators attempted to clarify this risk in a retrospective cohort study of 81,784 singleton deliveries taking place in the UK in the years 1968–1989. Excluding those caused by congenital anomalies there were 290 stillbirths, 113 of which were classified as explained, and 177 as unexplained. Mothers having a previous cesarean delivery tended to be older than the others, to have lower parity, to be of higher social class, and to have had previous adverse pregnancy outcomes.
Unadjusted hazard ratios for all stillbirths, those that were explained, and those unexplained were, respectively, 1.54 (95% confidence interval [CI], 1.04–2.29), 2.13 (95% CI, 1.22–3.72), and 1.19 (95% CI, 0.68–2.09). After adjusting for maternal age, parity, social class, previous adverse pregnancy outcomes, body mass index, and smoking status, the respective hazard ratios were 1.58 (95% CI, 0.95–2.63), 2.08 (95% CI, 1.00–4.31), and 1.24 (95% CI, 0.60–2.56). Stillbirths following a previous cesarean section were more likely than in the “unexposed” group to be explained, and they were likelier to be associated with abruption/hemorrhage or mechanical causes. It was estimated, using a hazard ratio of 1.58 to approximate the adjusted relative risk in a population with a section rate of 7.1% and an average stillbirth rate of 0.35%, that approximately 4% of all stillbirths in the population may be ascribed to previous cesarean delivery.
These findings affirm an increased risk of stillbirth in women previously having a cesarean section. The risk is most evident in cases of explained stillbirth. Further research is needed to determine whether this association is causal and, if so, what mechanisms might be involved.