The American College of Obstetricians and Gynecologists recommends that elective C-sections be performed no sooner than 39 full weeks of gestation. A recent research study funded by the National Institute of Child Health and Human Development (NICHD) found that 36% of C-sections were performed earlier than this benchmark. Babies delivered by C-sections before 39 weeks have been shown to have increased birth-related problems such as respiratory distress, lower blood sugar, infection or a need for intensive care.
http://www.usatoday.com/news/health/2009-01-07-C-section-elective_N.htm