First-time caesarean section births for women with no identified medical risks or complications have risen sharply in the United States, according to research published on Friday.

Professor Eugene Declercq, of Boston University School of Public Health in Massachusetts, said so called "no indicated risk" first caesareans rose by 67 percent between 1991 and 2001.

"The real surge comes after 1996, which coincides with a national increase in caesarean sections," Declercq said in an interview.

"For mothers with no indicated risk, about 5.5 percent had a first-time caesarean."

Declercq said the trend could be worrying because women who have a caesarean with their first baby are likely to have it with subsequent children, which can increase the risk of complications.

Declercq and his colleagues analyzed U.S. national birth certificate data on about 4 million births each year and created the new "no indicated risk" category to look at caesarean births in these women. It includes women who gave birth to single full-term babies who had a first-time caesarean but did not have any reported risks or complications, such as a very long labor, breech position of the baby or fetal distress which would necessitate it, listed on the birth certificate.

But the researchers said it is possible that there were other risks that could have been picked that were not listed.

First-time older mothers are much more likely to have a caesarean but the researchers said that even considering that there was a substantial increase.

"Mothers were 50 percent more likely in 2001 to have a 'no indicated risk' (caesarean), even after controlling for age, race, ethnicity, education, birth weight and parity," said Declercq, who reported the findings in the British Medical Journal.

But the researchers added that they did not know if the increase was due to patient choice, physician choice or a combination of the two.

The scientists said more research was needed to determine whether the risk of a "no indicated risk" primary caesarean would be offset by associated benefits.