Preterm births reached an all-time high in 2004, numbering half a million for the first time in the U.S., according to federal statistics released Tuesday.

The figures also show a continued rapid rise in the use of cesarean delivery among U.S. women. C-section rates rose 6 percent in 2004 and have gone up 40 percent since 1996, worrying some experts that not enough attention is being paid to the procedure’s consequences.

Joyce Martin, a researcher with the National Center for Health Statistics at the CDC, tells WebMD that preterm and low-weight births have been on a “slow but steady” rise for a decade.

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The Role of Multiple Births

Part of the reason is an increase in multiple births, which are more likely to be premature or low-weight, she says. Martin attributes rising multiple births to increased pregnancy rates among women in their 30s and 40s, who are more likely to give birth prematurely and more likely to use fertility drugs that can lead to twins.

“Other reasons are just not clear,” Martin says.

The report found a slight decrease in smoking among pregnant women, though 10.2 percent of women still smoked during their pregnancies in 2004.

At the same time, C-section rates reached 29.1 percent of all births in 2004, the highest number ever recorded in the United States. C-sections during women’s first pregnancies were also up.

Patients of all ethnic groups were more likely to have C-sections, suggesting that the procedure is increasingly popular with doctors.

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Why C-Sections Are Popular

Experts point to several possible factors -- including fashion -- boosting the popularity of C-sections among women in their first pregnancy.

“There isn’t any specific study that you can link the trend to,” says Eugene Declercq, PhD. Declercq is a C-section researcher and a professor of maternal and child health at the Boston University School of Public Health.

Doctors may be practicing more C-sections as a way to avoid liability lawsuits. More mothers may also be asking their doctors for C-sections because of the increased convenience of choosing the time of delivery and because of a desire for less painful childbirth, he says.

In addition, C-section has simply become part of popular culture. “When Britney Spears has an elective C-section, it’s all over the place,” Declercq says.

But if convenience is a factor, it may be misplaced, he warns. “I’m not sure how convenient it turns out to be for mothers because they’re in the hospital longer and it takes longer to recover” after C-sections,” Declercq tells WebMD.

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‘We’ve Gone Too Far’

Sarah Kilpatrick, MD, the chief of obstetrics and gynecology at the University of Illinois at Chicago, called the increasing C-sections rates “very disturbing.”

Kilpatrick agrees that more doctors are caving to patient demands for elective cesarean births.

But the trend may be ignoring evidence that repeated C-sections increase the risk of dangerous placental abnormalities in later pregnancies. More first-time cesareans are now increasing the baseline rate of repeat surgeries later, each of which carries progressively higher risks to both mother and newborn, says Kilpatrick, who is vice chairwoman of the practice committee of the American College of Obstetricians and Gynecologists.

“I don’t think anyone has really thought to look at the long-term consequences of this,” she tells WebMD. “I think it’s clear that we’ve gone too far.”

By Todd Zwillich, reviewed by Louise Chang, MD

SOURCES: CDC: "Preliminary Births for 2004: Infant and Maternal Health." Joyce A. Martin, National Center for Health Statistics, CDC. Eugene Declercq, PhD, professor, maternal and child health, Boston University. Sarah Kilpatrick, MD, head, department of obstetrics and gynecology, University of Illinois, Chicago.