More than a quarter of babies born in the United States in 2003 were delivered by Cesarean section (search), the highest rate on record, according to a government report released Tuesday.

Birth rates for teenagers continued their steady decline last year while increasing for women age 35 to 44, the report from the National Center for Health Statistics said. The agency is part of the Centers for Disease Control and Prevention.

Nearly 4.1 million births were recorded in the United States in 2003, a slight increase over 2002. Roughly 1.13 million, or 27.6 percent, were Cesarean deliveries. The rate is up by a third since 1996, said the report, which is a preliminary look at U.S. births last year.

A Cesarean section is major abdominal surgery with serious potential side effects. The report does not distinguish between those that were medically necessary and those that were elective.

The question of whether it should be performed when natural childbirth poses no threat to either mother or baby is controversial among obstetricians.

One unexplained trend in the annual report is the continued increase in the rate of premature and low birthweight babies even though the teen birth rate dropped, fewer women were smoking while pregnant, and more women were getting timely prenatal care.

The rate of babies born after less than 37 weeks of gestation rose slightly to 12.3 percent, the report said, and those weighing less than 5.5 pounds increased slightly to 7.9 percent last year.

Some of the rise in these early births can be tied to the increasing number of older mothers, who naturally and through fertility treatments are more likely to have twins and triplets. These babies are more likely to be born early and weigh less, said Joyce Martin, an epidemiologist and author of the report.

"But it's important to note that the increase in preterm and low birthweight is not restricted to older moms and for women just having singletons. So something else is going on here, too," Martin said.

Among other statistics in the report:

— Births to unmarried mothers rose slightly.

— Women of Hispanic origin had the highest birth rate, 22.9 per 1,000 people, compared to the overall rate of 14.1.

— Two teenage girls younger than 15 gave birth to at least their fourth child.

— There were 1,512 first-time mothers between the ages of 45 and 54.