TORONTO — Women whose first babies are delivered by cesarean section face an elevated risk of placenta previa and placental abruption in their second pregnancies. And with two previous cesarean deliveries the risk of placenta previa is increased further in the third pregnancy, according to a study by Dr. Darios Getahun of the Robert Wood Johnson Medical School in New Brunswick, N.J., and his colleagues.
The study, which was recently published (Obstet. Gynecol. 2006;107:771–8), was presented as a poster at the annual meeting of the Society for Gynecologic Investigation.
“Although cesarean section has previously been reported as a risk factor for placenta previa, it has not been previously associated with abruption,” Dr. Getahun said in an interview at the meeting. “Cesarean section causes scarring of the uterine wall, with the result that placentation may not be optimal. That's why it may be leading to abruption,” he explained.
The study included a cohort of women from the Missouri longitudinally linked live birth and fetal death data files. Singleton births were analyzed for 156,475 women whose first two consecutive births occurred within the study period of 1989–1997, and 31,102 women whose first three consecutive births occurred within that period.
Among 40,472 women whose first delivery was by cesarean section, the relative risk of placenta previa was 1.5, and that of placental abruption was 1.3 in the second pregnancy, compared with women whose first delivery was vaginal.
There was a dose response noted for the risk of placenta previa, but not for placental abruption risk. Therefore, when both the first and second deliveries were by cesarean section, the risk of placenta previa doubled in the third pregnancy, but the risk of placental abruption did not increase further, compared with women whose first two deliveries were vaginal.
The interval between pregnancies also was analyzed, and the study found that for cesarean deliveries, but not vaginal ones, an interval of less than 1 year was associated with a relative risk of 1.7 for placenta previa and 1.5 for placental abruption.