The analysis was based on data collected by 19 academic medical centers for the National Institute of Child Health and Human Development, part of the NIH.
Researchers identified women who delivered singleton babies by C-section between 1999 and 2002, even though they had no medical reason for doing so.
Compared with babies born after 39 weeks, babies delivered after 37 weeks were 4.2 times more likely to suffer from respiratory distress syndrome (3.7% of births versus 0.9%) and about three times more likely to be treated for hypoglycemia (2.4% versus 0.7%) or newborn sepsis (7% versus 2.5%).
They also had more than double the risk of being admitted to a neonatal intensive care unit and were almost three times as likely to require more than five days of hospitalization.
Babies born after 38 weeks had twice the risk of respiratory distress syndrome compared with those born after 39 weeks and were 30% to 80% more likely to have other serious complications, the study found.
The results suggest that the seven days during the 40th week of pregnancy is the safest time to schedule an elective C-section, Thorp said. That window is "smaller and more tightly bound" than previously thought, he said.
Many patients should be able to shift their deliveries into that window without too much trouble, D'Alton said. In the study, 52% of the early deliveries occurred in the last three days before reaching the 39-week threshold.
The researchers also found that the risks rose again after 40 weeks of pregnancy, said Dr. Alan Tita, an obstetrician at the University of Alabama at Birmingham who led the study.
"We were surprised," Tita said. "However, very few planning an elective cesarean would wait beyond 39 to 40 weeks."
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karen.kaplan@latimes.com
