Thirty-six percent of elective C-sections are performed too early. This can increase newborns' risk of respiratory distress and other problems.

A study looked at more than 13,000 elective repeat C-sections. The American College of Obstetricians and Gynecologists recommends that elective C-sections be performed no sooner than 39 full weeks of gestation. However, more than a third of the C-sections were performed earlier than that.

About 10 percent of the babies had respiratory distress, low blood sugar, infection or need for a respirator or intensive care. Those born at 37 weeks were twice as likely to have a problem, and those born at 38 weeks 50 percent more likely to have a problem, than those born at 39 weeks.

Elective repeat C-sections have become increasingly common, and currently represent about 40 percent of the 1.3 million cesareans performed each year in the U.S.