Those delivered earlier had more complications, including breathing problems, even though they were full term, the researchers reported in Thursday's New England Journal of Medicine. Even just a few days made a difference, they said.
"We now know that there are some risks of planning and doing a delivery before 39 weeks," Dr. Rebecca Brightman, an OB/GYN at Mt. Sinai Medical Center in New York, told CBS News.
The findings offer important guidance to the growing number of women who face planned C-sections. And the study supports recommendations that elective C-sections be scheduled after 39 weeks unless tests show the fetus's lungs are fully mature. Due dates are set at 40 weeks' gestation and a fetus is full term at 37 weeks.
"Take your due date and subtract seven and any one of those seven days is fine," said one of the researchers, Dr. John Thorp of the University of North Carolina at Chapel Hill School of Medicine.
He delivered a healthy baby girl on Tuesday after persuading her mother to wait a few more days for a C-section, even though last week would have been more convenient for both mother and doctor.
"We bit the bullet and did it at the right time," said Thorp.
The rate of caesarean sections in the United States is at an all-time high, accounting for about 31 per cent of births. There are lots of reasons: older moms, multiple births, the threat of malpractice lawsuits, the preference of mothers and doctors and the risks of having a vaginal birth after a previous caesarean.
A U.S. government panel in 2006 urged women not to seek a C-section without a medical reason; surgery brings risks and babies born by caesarean have a greater chance for respiratory problems.
"Truth be told, of you end up with a baby with a problem and that baby ends up in the ICU and you can't leave the hospital with that baby, who cares about the convenience?" Brightman told CBS News. "I think when people realize that there are some risks, then they're willing to say 'You know what, I'm willing to wait until 39 weeks.'"
In the new study, the researchers examined a C-section registry from 19 medical centers to see how many of the surgeries were being done before the recommended 39 weeks and if the timing made a difference in the risk of complications.
They focused on 13,258 women who had a single child with a planned caesarean and had previously given birth the same way. More than a third of the C-sections were performed before 39 weeks.
Those delivered at 37 weeks were twice as likely to have health problems, including breathing troubles, infections, low blood sugar or the need for intensive care. Fifteen per cent of those born at 37 weeks and 11 per cent born at 38 weeks had complications, compared to eight per cent of the babies delivered at 39 weeks. The only death was an infant born at 39 weeks.
The biggest difference was in breathing problems, with a fourfold increase for those born at 37 weeks compared to 39 weeks. Babies born by C-section already have a higher risk of breathing trouble than those born vaginally: labor helps clear the lungs of fluid.
The risk of complications also increased for births after 41 weeks, but there were few births in that category, the researchers said.
Lead investigator Dr. Alan Tita of the University of Alabama at Birmingham said the researchers didn't know the reasons behind the chosen delivery dates. They speculated that some mothers might have wanted to deliver as soon as the fetus reached full term, or an earlier time may have been more convenient for the mother or doctor.
Dr. Michael Greene of Massachusetts General Hospital in Boston noted the research showed that there's even an increased risk of complications in the last few days before the 39th week. That's something most doctors wouldn't suspect, he said.
"I generally try to wait to 39 weeks, although I confess that I'm as guilty as anybody else with a busy practice and scheduling being what it is," said Greene, who wrote an accompanying editorial in the journal. "I really hadn't thought much about it until now."
The study only looked at live births and didn't account for the small risk of the fetus dying while waiting to reach 39 weeks, Greene said. That risk has been estimated at up to one in 1,000, he said.
Thorp's patient in Chapel Hill, Shannon Eubanks, said she was glad that she held off a few days to reach the 39-week threshold before having her daughter, Kathleen Conley Eubanks. Her first child, two-year-old Charlie, was born by C-section.
"It was hard to wait," said Eubanks, business manager of the political science department at the university. "I was very over being pregnant and ready to get the show on the road."
And she had another reason for wanting to deliver last week.
As an accountant, "It just killed me not to have this kid in 2008" to get the tax deduction, Eubanks said hours after delivering.