Babies delivered by cesarean section a week or two before the recommended 39 weeks of pregnancy face a heightened risk of respiratory problems and other complications, researchers report in the Jan. 8 New England Journal of Medicine. Being born late isn’t good either, the study finds.
Scientists consider normal human gestation to be 39 to 40 weeks, which is about nine months. Doctors have adopted some leeway in this calculation, considering a baby to be “full term” if delivered at 37 weeks or later.
But past research had raised questions about early deliveries, and practice guidelines urge women to hang on until 39 or 40 weeks before delivering. Mainly, this extra time allows for full development of the fetus’ lungs.
In the new study, obstetric gynecologist Alan Tita of the University of Alabama at Birmingham and his colleagues collected birthing data at 19 medical facilities in the United States. The team identified more than 13,000 cases in which a woman had delivered by elective (nonemergency) cesarean section at 37 weeks or later, having had a previous cesarean delivery at some point. The researchers excluded from the analysis women who had medical problems, had an emergency cesarean or had already begun labor before undergoing a cesarean.
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Roughly one-third of these women delivered before reaching the 39-week point in the pregnancy. The researchers found that 15 percent of babies delivered at 37 weeks had a complication, compared with 8 percent of those delivered at 39 weeks. Complications included respiratory problems, low blood sugar and a blood infection, or the need to go to the intensive care unit, get resuscitated, put on a ventilator or stay in the hospital more than five days.
Common complications were respiratory distress and transient tachypnea. Infants with these complications struggle to breathe and have trouble clearing fluid from their lungs. One or the other of these problems showed up in the 37-week group more than twice as often as in the 39-week babies.
Meanwhile, the researchers found that 11 percent of babies born at 38 weeks — one week short of nine months —had complications, a rate somewhat higher than the 8 percent of the 39-week group.
Those born at 40 weeks were not more likely to have problems, but babies born after 41 or 42 weeks faced risks similar to those born at 38 and 37 weeks, respectively.
A closer look at these women shows that those delivering earlier were more likely to be married, white and privately insured than those delivering at 39 weeks or later, says obstetric gynecologist Michael Greene of Harvard Medical School and Massachusetts General Hospital in Boston, who didn’t participate in the study. The early deliverers may have placed a premium on having their own doctors perform the cesarean, which requires planning and scheduling, he says.
The risks of such early deliveries are now clearer, Tita says. “This study brings some of these problems to the fore. Hopefully, with this publication, some of these practices will change,” he says.
But there remains at least one major confounding factor in all this: The risk of stillbirth is greatest at 39 weeks or more. Roughly one in 1,000 full-term births end in stillbirth. These cases of fetal death can be traced to many factors, including bacterial infections, umbilical cord problems, trauma, drug or alcohol consumption by the mother or high blood pressure in the mother.
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Biology also plays a role in stillbirth risk. As a fetus grows, its metabolic needs increase and it demands more nourishment and oxygen, says Bryan Richardson, an obstetric gynecologist at the University of Western Ontario in London, Canada. As the fetus begins to tax its nutrient supply, he says, “its tolerance for an emergency lessens,” and that increases the risk of stillbirth should a problem strike very late in pregnancy.
Delivering a viable fetus at 37 or 38 weeks eliminates the risk of stillbirth occurring later. But it remains unknown whether avoiding the slight risk of stillbirth outweighs the other risks shown in this study that result from early delivery, says Greene. “This is interesting and useful information, but the stillbirth risk is not accounted for,” he concludes.