"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 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.
The rate of caesarean sections in the United States is at an all-time high, accounting for about 31 percent of births.
In the study, researchers led by Dr. Alan Tita of the University of Alabama at Birmingham examined a C-section registry from 19 academic medical centers to see how many of the surgeries were being done before the recommended 39 weeks and whether the timing made a difference.
The biggest difference was in breathing problems, with a fourfold increase for those born at 37 weeks compared with 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.
Dr. Michael Greene, of Massachusetts General Hospital, said the research showed there's an increased risk of complications in the last few days before the 39th week, which most doctors wouldn't suspect.
"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. Thorp's patient in Chapel Hill, Shannon Eubanks, was glad she held off a few days to reach the 39-week threshold before having her daughter, Kathleen. Her first child, Charlie, 2, was born by C-section.
"It was hard to wait," said Eubanks, a business manager 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.
