Babies delivered by Caesarean section at 37 weeks were 4.2 times more likely than those delivered at 39 weeks to suffer from respiratory distress syndrome, and 3.3 times more likely to be treated for hypoglycemia, 2.9 times more likely to develop an infection or to be suspected of having an infection, and more than twice as likely to be hospitalized for five or more days or to be admitted to the neonatal intensive care unit.

Babies delivered by Caesarean section at 38 weeks were 2.1 times more likely than those delivered at 39 weeks to suffer respiratory distress syndrome, 1.8 times more likely to be hospitalized for five or more days, and 1.7 times more likely to have an infection or be suspected of having an infection. Babies delivered by Caesarean at 38 weeks had a 30 percent increased risk of hypoglycemia and a 50 percent increased risk of being admitted to the neonatal intensive care unit, compared to those delivered at 39 weeks.

“Faced with the likelihood that there is no other problem, I think it’s prudent to wait until 39 weeks to avoid subjecting these babies to adverse outcomes,” Dr. Tita said. He added that he did not think the results would be different for women undergoing a first elective Caesarean section, versus the repeat Caesareans looked at in this study.

Reactions to the study were mixed. Dr. Michael Greene, director of obstetrics at Massachusetts General Hospital, who wrote an editorial that accompanied the article, said the paper quantifies differences in risk that many physicians may not be aware of.

“I would bet if you ask the first 10 obstetricians on the street if they thought it would make a difference to deliver a baby in the second half of the 38th week and the first half of the 39th week, they’d say, ’Nah, it’s not a big difference,’ ” he said. “This study, because of its size and numbers, is able to say, Yeah, it does make a difference.’ ”

At the same time, he said the study only reviewed live births and did not weigh the risk of a potential stillbirth that could occur with a longer pregnancy against the risk of a complication resulting from an earlier Caesarean.

“The apparent simple message from this study is too simplistic,” he said. “There is a tiny, small but real increase in stillbirths late in pregnancy for women who had a prior Caesarean section, as opposed to women with no scar in their uterus” from a previous Caesarean section.