The rate at which U.S. women are giving birth through Caesarean sections has edged down for the first time in more than a decade, according to federal data released Thursday, reversing a long trend that had raised alarm about the health and well-being of new mothers and newborn babies.

The proportion of children delivered through Caesarean receded from 32.9 percent in 2009 to 32.8 percent in 2010, a preliminary analysis of the latest annual statistics collected by the National Center for Health Statistics showed.

Although tiny, the shift comes after Caesareans had risen steadily and seemingly inexorably between 1996 to 2009, stirring deep concerns because the surgical deliveries can be risky for the mother and child, make it tougher for new mothers to recover and are more expensive.

The reason for the steady increase has been the subject of intense study and debate. But experts say it was due to more women having babies later in life, doctors spotting more problem pregnancies earlier and more busy women choosing the convenience of knowing when they would deliver, among other factors. Doctors are also reluctant to try a vaginal delivery in women who previously had a C-section, fearing complications and malpractice lawsuits.

The rise had been slowing in recent years. The reason for that too remains unclear but could be due at least in part to the increased attention the issue has been getting.

“I think the message that C-sections have increased risks to mother (both short term and in subsequent pregnancies) is getting out to providers and patients as well,” George Macones of the American College of Obstetricians and Gynecologists wrote in an e-mail. “I think this is very welcome news, since we know that C-sections are associated with increased complications for mom. I hope that trend continues.”

One key factor may be a decreasein the number of women giving birth totheir first child, according to Michael F. Greene, who heads the obstetrics department at the Massachusetts General Hospital in Boston.

“For women having their first babies, the rate is way higher than the rate for women having their second baby,” Greene said. “If you had one baby normally you are much less likely to require a C-section the second time around.”

The same report also showed the total number of children being born in the United States is still declining.

Among other milestones, 2010 was the third year in a row that fewer teens had babies. The teen birth rate fell 9 percent between 2009 and 2010, dropping to 34.3 births per 1,000 youths ages 15 to 19. The rate has now declined for 17 of the past 19 years and reached another record low, having fallen 44 percent since 1991.

The resumption of the fall in teen births has been welcomed by many public health advocates. A two-year increase in 2006 and 2007 had spurred alarm and intense debate over comprehensive sex education vs. focusing on encouraging abstinence until marriage.

“Progress has been nothing short of remarkable,” said Sarah Brown of the National Campaign to Prevent Teen and Unplanned Pregnancy. “Clearly, the teens have changed their behavior. The magic formula of less sex and more contraception is responsible for this great good news. Why are teens being more careful? I suspect it is due to the recession, more media attention to this issue ... and more attention to and investment in evidence-based programs.”

Researchers have speculated that the recession has made women more hesitant to have children out of concern about supporting them.

The fall in births among teens reflected a continuation of a larger trend. The total number of births declined 3 percent, driving the total fertility rate down to 1.9, the lowest since 1987. A total fertility rate of 2.1 is considered the “replacement rate” — the pace necessary to sustain the population. The U.S. total fertility rate had risen above the replacement rate in 2006 and 2007, but then fell back again.

The birth rate among women in their early 20s, late 20s and 30s all fell as well. The only age group showing an increase in births was women in their early 40s, whose rate rose by 2 percent, hitting the highest rate since 1967.

Preterm births declined for the fourth year, to just under 12 percent, a 6 percent drop from 2009.

“We have made great strides in the last few years in terms of interventions to reduce preterm birth, but more importantly on education of physicians and patients about risks of later preterm births,” Macones said. “All of that work seems to be paying off.”