The report of the Agency for Healthcare Research and Quality, part of the U.S. Department of Health and Human Services, said the proportion had been 1 in 5 in 1995.

The report also said that about 1.3 million women gave birth via Caesarean section in 2005 -- a 38 percent increase over the 800,000 Caesarean sections performed in 1995.

The increase occurred as vaginal deliveries among women who gave birth in hospitals declined from about 3 million in 1995 to 2.9 million in 2005 -- a decrease of 3 percent. The sharpest decline in vaginal deliveries in hospitals was among women who had previously given birth by Caesarean section.

Hospitals charged $21.3 billion for patient stays involving vaginal delivery in 2005 and $17.4 billion for those involving birth by Caesarean section, the report said.

Cesarean section is a surgical method usually performed when a vaginal delivery would put the baby's or mother's life or health at risk. However, the procedure is performed during births that would otherwise have been normal, the report said.