Women who have their first baby by Caesarean section are at significantly higher risk of losing their next baby to an unexplained stillbirth before going into labor, according to a new study.
An expert not associated with the study, published in The Lancet medical journal, said the research is important because it suggests hospitals that tend to do a lot of Caesareans might need to consider strategies to reduce them.
The study led by Gordon Smith of the Department of Obstetrics and Gynecology at Cambridge University looked at 120,633 second births of single babies in Scotland. It found that among 17,754 women who had delivered their first child by Caesarean section, there were 68 stillbirths before labor - or 239 per 10,000 women per week.
Among 102,879 who had delivered their first child vaginally, there were 244 stillbirths before labor, the equivalent of 144 per 10,000 women per week.
The researchers found that the risk of unexplained stillbirth associated with previous Caesarean delivery differed significantly with the age of the fetus: the most dangerous time was at 34 weeks’ gestation.
The risk was not reduced by factors such as the mother’s age or the outcome of her first pregnancy, the researchers found.
"The absolute risk of unexplained stillbirth at or after 39 weeks’ gestation was 1.1 per 1000 women who had had a previous Caesarean section and 0.5 per 1000 in those who had not," the study said.
"The difference was due mostly to an excess of unexplained stillbirths among women previously delivered by Caesarean section."
The researchers concluded that "delivery by Caesarean section in the first pregnancy could increase the risk of unexplained stillbirth in the second."
Previous studies have indicated that women who have had a Caesarean and who later attempt to deliver by labor are more likely to suffer a uterine rupture - a splitting open of the womb that is life-threatening for both mother and infant - than women who have another Caesarean delivery.
The researchers said that in women with one previous Caesarean delivery, the risk of unexplained stillbirth before labor at or after 39 weeks’ gestation "is about double the risk" of stillbirth or neonatal death from uterine rupture during labor.
The risk of uterine rupture during childbirth is estimated at around 0.45 per 1,000 births; the risk of unexplained stillbirth at around 39 weeks among women with one previous Caesarean is 1.06 per 1,000, they said.
Judith M. Lumley of the Centre for the Study of Mothers’ and Children’s Health in Victoria, Australia, said the study "could redefine the nature of the debate about the place of Caesarean delivery in maternity care."
Writing separately in The Lancet, Lumley said it was already known that previous Caesarean delivery is associated with an increase in subsequent problems like placenta praevia, where the mother’s placenta becomes detached too early, and ectopic pregnancy, where a fetus implants outside the womb.
"But an association of previous Caesarean section with an increase in fetal death at and after 34 weeks’ gestation was not known," she wrote.
She said one implication of the study "might be that maternity hospitals where Caesarean first births are commoner than in other local settings need to think more seriously about strategies for reducing the number of Caesarean sections."
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