The magic number is 39 weeks.

MedPage Today reports on a NEJM study that looked at women who underwent a repeat Caesarean delivery. It found that infants who delivered early, defined as less than 39 weeks, had a markedly increased risk of adverse outcomes, which included “adverse respiratory outcomes, need for mechanical ventilation, newborn sepsis, hypoglycemia, admission to the neonatal ICU, and hospitalization for five days or longer.”

40 percent of elective Caesarean deliveries are repeat procedures, and this number is sure to go up as the rate of initial C-sections rises due to malpractice fears and a physician payment system that favors procedures.

You can’t wait too long however, as the study also found the rate of stillbirth increases if delivery is delayed past 40 weeks.

So when counseling expectant mothers anticipating a C-section, 39 weeks seems to be the ideal delivery time.