While c-sections have saved the lives of "countless" women and babies, and the risk of infant death is still very low, it is crucial to determine the reasons for the higher infant mortality seen with c-section, because the rates of this surgery are becoming increasingly common, Dr. Marian F. MacDorman of the National Center for Health Statistics at the Centers for Disease Control in Hyattsville, Maryland and colleagues conclude.

Rates of cesarean have risen steadily in the US, from 14.6 percent of all first-time births in 1996, to 20.6 percent in 2004, MacDorman's group notes in the September issue of Birth.

Since the US began gathering data on c-sections in 1989, MacDorman and her team note, a greater risk of death has been seen among infants born via the procedure, but researchers have generally assumed that this was because these infants were more likely to die due to other causes.

To investigate whether the c-section itself might somehow be a factor in infant deaths, the researchers looked at data from more than 5.8 million births to US women between 1998 and 2001. All of the women were at "no indicated risk" for a C-section, meaning the infant was a singleton, full-term, in a head-down position, and no other medical risk factors or delivery complications were indicated on the child's birth certificate.

MacDorman and her colleagues had previously identified a 49-percent increase in c-section rates between 1996 and 2001 among women in this "no risk" category.

The risk of death in the first 28 days of life was 1.77 per 1,000 live births among women who had c-sections, compared to 0.62 per 1,000 for women who delivered vaginally. Even after analyzing the various causes of infant death, the researchers could find no clear explanation for the difference.

"Understanding the causes of these differentials is important, given the rapid growth in the number of primary cesareans without a reported medical indication," they conclude.

SOURCE: Birth, September 2006