Last month yet another study appeared reporting that compared with first vaginal birth, first cesarean increased the likelihood of late antepartum fetal death in the next pregnancy. The study encompassed 10,712 women with one prior birth who had pregnancy duration of 34 weeks or more and were carrying a single, normally-formed fetus. With first delivery via cesarean (22%), the fetal death rate at or beyond 34 weeks' gestation in the next pregnancy was 2.5 per 1000 compared with 0.5 per 1000 with first birth vaginally, or 2 more late antepartum fetal deaths per 1000 with first delivery via cesarean surgery.

I say 'yet another study' because it joins eight others. Six of the eight, one each in Scotland, England, Germany, and Canada and two in Australia, one in South Australia and the other in New South Wales, also reported more late fetal deaths with first cesarean delivery. In two of them, the difference failed to achieve statistical significance, meaning the difference may have been due to chance, but the number of women having a first cesarean was too small to reliably detect a difference. A third study among the six did not perform a significance calculation. The seventh study, conducted in Missouri, reported an excess among black women but not white women. Mortality rates varied substantially from study to study, but excesses with prior cesarean were similar, ranging from 0.3 to 1.6 per 1000 (mean 1.1 per 1000). The eighth study, a U.S. national study, reported no difference (0.7 per 1000 first cesarean delivery vs. 0.8 per 1000 first birth vaginal) in women with one prior birth, no underlying medical conditions, and a fetus with no structural or chromosomal abnormalities. The gap actually may be wider than appears. Some of the studies restricted analysis to unexplained deaths, which excluded deaths secondary to placenta previa, and accreta and placental abruption, all of which are associated with prior cesarean.

The consistency of this finding is compelling, but you may be thinking that it shouldn't be surprising because some of the reasons that may lead to cesarean in the first pregnancy would increase the risk of fetal demise in the next pregnancy. Ah, but unlike the other studies, which used population databases, this one was conducted at a single hospital, which means investigators could explore the effect of confounding factors. They found that the association remained statistically significant after controlling for maternal age, height, weight, hypertension, and diabetes, and it strengthened when they confined analysis to women known to have first births to a full-term live infant (n = 4425): 6 per 1000 with first delivery by cesarean versus 1 per 1000 with first birth vaginal, or 5 more late antepartum deaths per 1000 in women with first cesarean delivery in this subgroup. The cause of the excess is unknown, but it would appear that a scarred uterus becomes a less hospitable environment for pregnancy.

Certainly, this risk should not deter performing a cesarean when the health of mother or baby is at stake or everything has been tried, but it seems unlikely that the baby can be born vaginally. However, with one in three first time mothers delivering via cesarean surgery, for many cesareans, clearly, this is not the case. Many cesareans could be prevented with better labor management and by having more patience. As the ninth study concludes, 'Our findings reinforce the importance of considering the impact cesarean birth may have on future pregnancies when making decisions regarding method of birth' (p. 16). Amen to that.

Published: March 20, 2012

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