Dr. Alan Fleischman, medical director and senior vice president of the March of Dimes infant health advocacy group, voiced concern that a sizable portion of these C-section deliveries may be medically unnecessary.
Premature babies are at greater risk for a number of medical and developmental problems such as troubled breathing, bleeding in the brain, birth defects and death. Premature birth is defined as delivery before the 37th week of pregnancy, rather than the typical 40 weeks.
Researchers at the U.S. Centers for Disease Control and Prevention and the March of Dimes compared single births -- not twins or other multiple births, which are at an increased risk for pre-term birth -- in 1996 and 2004. The rate of premature births rose by about 10 percent in that period, they said.
The number of premature births rose from 354,997 in 1996 to 414,054 in 2004, the study published in the journal Clinics in Perinatology showed.
"When one looks at the numbers carefully, there was an increase of 60,000 who were pre-term, and 92 percent of them were by Caesarean section," Fleischman said.
The increase comes amid an ongoing controversy over whether some doctors are performing C-sections because they fear being sued if they do not and whether some women are opting for medically unnecessary C-sections out of convenience.
A C-section -- delivery of a baby through a surgical abdominal incision -- is advised when a vaginal birth is not possible or is unsafe for the mother or child.
"The increase in pre-term births is really being driven by the Caesarean section rate, and really demands good research to sort out what percent of those are not medically indicated deliveries," Fleischman said in a telephone interview.
"My gut tells me its significant, but I can't give you an estimate and a percent," Fleischman added.
There has been considerable controversy in the medical field over a dramatic increase in C-section births in the United States and some other countries in recent decades.
In the United States, 5 percent of babies in 1970 were born by C-section. In 2006, the figure was about 31 percent.
Meanwhile, the percentage of babies born prematurely also has been rising -- increasing more than 30 percent since the early 1980s to about 13 percent of all births. Being born pre-term gives an infant less time to develop in the womb.
Editing by Anthony Boadle
© 2008 Reuters