Women who give birth by Caesarean section run twice the risk of having a stillbirth at the next pregnancy, researchers reported yesterday.

Although the risk of a later stillbirth remains low, at about one in 1,100, doctors say the finding could redefine the nature of the debate about Caesareans, which have more than doubled in the past 20 years. Concern about Caesareans has previously focused on the immediate potential risks to mother and child. Problems with the development of the placenta in subsequent pregnancies have been noted but this is the first time Caesareans have been linked with an increased risk of stillbirth.

Most Caesareans are carried out to avoid the complications of a vaginal delivery, especially involving a breech baby, and the small additional risk of a stillbirth in a future pregnancy is unlikely to affect the decisions. But an increasing number are performed at the request of the mother, and not for a clinical reason. A survey in 2001 by the Royal College of Obstetricians and Gynaecologists found 7 per cent of Caesareans were at maternal request, amounting to about 2,400 a year.

Nowadays, more than 20 per cent of births are Caesarean, up from 9 per cent in 1980, and obstetricians say pressure from patients for Caesareans is intensifying.

Gordon Smith, from the department of obstetrics at the University of Cambridge, and colleagues who carried out the study, said in The Lancet: "If women are being counselled about Caesarean birth with no clear obstetric advantage, such as Caesarean section for maternal request, the possible effect on the risk of unexplained stillbirth in future pregnancies should be discussed."

The authors studied data for 120,000 births in Scotland between 1992 and 1998, and found the risk of a stillbirth from 34 weeks gestation was 1.77 per 1,000 women who had had a previous Caesarean compared with 0.89 per 1,000 for other women. The risks of a breech baby dying in a vaginal birth are put at 8.9 in 1,000 and the risk of a later stillbirth are said to be less than one in 1,000. Therefore, the new findings are "unlikely to influence the decision to have a Caesarean in a breech pregnancy," they said.

But the findings could influence the choices women make in subsequent pregnancies ­ and increase Caesareans. If the risk of a stillbirth in a subsequent pregnancy is increased from the 34th week onwards, women may decide they want to deliver the baby early by Caesarean. Many women are advised to have a second Caesarean because of the small extra risk of the womb rupturing along the old scar, which is put at 0.45 per 1,000 deliveries.

The researchers said the risk of a stillbirth from the 39th week of pregnancy was greater than double this risk in women who had previously had a Caesarean, at 1.06 per 1,000. "The current data suggest that an additional benefit of planned repeat Caesarean delivery at 39 weeks gestation may be to reduce the risk of unexplained stillbirth." They said their results cannot be explained by any of the usual factors affecting stillbirths, such as smoking, social deprivation or the age of the mother.