A new study of birth certificates looks at trends in cesarean births in the United States.

What the researchers wanted to know: How often are caesareans done for women with what the writers call "no indicated risk"?

What they did: The writers looked at national birth certificate data on about 4 million births a year between 1991 and 2001. They were looking for cesareans done by choice (before labor started) in women with "no indicated risk." But since they were using only birth certificates, the risks that could be indicated were limited to a multiple birth (twins or more); the baby pointing down with anything but the top of the head (for example, a breech presentation); a baby being delivered early for any reason; and any other medical risk factors for the mother or complications of labor or delivery that happened to be recorded on the birth certificate.

What they found: Between 1991 and 2001, C-sections done under these conditions became more common. Women with no indicated risk factors were more likely to have a cesarean if they were in their 40s than if they were in their early 20s.

What the study means to you: If there are tens of thousands of women having unnecessary C-sections (and doctors providing them), that's a big deal. But these women could have had risk factors that didn't make it onto their offspring's birth certificates.

Caveats: The authors didn't examine women's medical records to find other factors that might have led doctors and mothers to choose a C-section. But they point out that there's no reason for those other factors to increase and cause the trend seen here.

Find out more: A history of the cesarean, from an exhibit at the National Library of Medicine

Read the article: Declercq, E., Menacker, F., and M. MacDorman. "Rise in 'No Indicated Risk' Primary Caesareans in the United States, 1991–2001: Cross Sectional Analysis." British Medical Journal. Published online Nov. 19, 2004.

Article online: http://bmj.bmjjournals.com