Higher Caesarean rates among older women cannot be explained by their increased risk of medical complications, a study has found.

The finding could mean many older women are undergoing major surgery unnecessarily.

First-time mums over 40 are 14 times more likely to have an elective Caesarean than those under 30.

Scientists also found that the likelihood that a woman would have a Caesarean increased gradually throughout her 30s.

It had been assumed that the risk of complications associated with older mothers explained the high number of operations carried out.

The observed relation between maternal age and Caesarean section cannot be explained by the obstetric complications we considered

Aberdeen Maternity Hospital researchers

But the rise in the Caesarean rate outstripped the increase in the risk of medical complications, scientists found.

The analysis, detailed in the British Medical Journal, looked at 23,806 births between 1988 and 1997, details of which were taken from the Aberdeen Maternal and Neonatal Databank.

The ratio of Caesarean to natural delivery births for women over 30 was compared to that for those in their 20s.

Scientists found that women aged 30 and 31 were twice as likely to have an emergency operation than those under 30.

Women over 40 who were already mothers were three times as likely.

Women of 38 or over were three to four times more likely to have a Caesarean than women in their 20s.

Birth trends

More than one in five births is now a Caesarean delivery.

But it is known women who have them take longer to recover than those who give birth naturally, and they may be more prone to bleeding, and even to infertility.

Experts also warn that because a Caesarean is a major operation, it has significant risks associated with it

Jacqueline Bell from Aberdeen Maternity Hospital, who led the research team, told BBC News Online: "People have suggested that obstetricians have a lower threshold of intervention in older women, and that older women themselves prefer Caesareans.

"But that is all speculation and this study has shown that there's a question mark, and that people need to do more research."

The researchers wrote: "The observed relation between maternal age and Caesarean section cannot be explained by the obstetric complications we considered.

"They add to previous findings by including greater numbers of obstetric complications and deliveries in the analysis."

They added: "This raises the question of why rates for Caesarean sections are high among older mothers, and whether they may be explained by physiological or other factors."

Audit

The Royal College of Obstetricians and Gynaecologists (RCOG), the Royal College of Midwives, the Royal College of Anaesthetists and the National Childbirth Trust are carrying out a national audit of Caesarean sections, due to be published in October this year.

Previous studies have shown four out of 10 maternity units have a Caesarean rate of 20% or more.

The World Health Organization says there is no clinical evidence to support a rate of over 15%.

An RCOG spokesman said the new study would look at how many Caesarean sections were taking place, how it was decided to do them, and what women think of Caesareans.

He said: "The audit may go someway to answer the questions raised in the BMJ article."

A spokesman for the Royal College of Midwives said: "We want to underline the fact that Caesareans involve major abdominal surgery and, as is the case with any operation, it will carry specific risks in terms of morbidity and mortality.

"In order to justify taking these risks, there have to be clinical benefits to the mother and the child."