Study: The Health Risks of Late Preterm Births

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In the largest study of its kind, researchers find that the risk of severe breathing problems rises significantly in babies born prematurely, even those born in the so-called late preterm period.

Health experts consider babies born at or after 37 weeks' gestation to be full term, and those born between 34 weeks and 37 weeks to be late preterm. (Preterm is defined as less than 34 weeks' gestation.) Many previous studies have shown that compared with full-term babies, those who are born too early are at higher risk of dying shortly after delivery and are more likely to suffer neonatal complications that require lengthy stays in the hospital.

In the new study, Dr. Judith Hibbard at University of Illinois also found that babies born at 34 weeks were 40 times more likely to have respiratory distress syndrome, a breathing difficulty that often requires a ventilator, than babies born at 38 weeks or later.

Even at 37 weeks' gestation — the point at which mothers may ask for an elective Cesarean section or induced delivery — babies are three times more likely than full-term infants to have respiratory abnormalities at birth. "That's a remarkably increased risk," notes Hibbard, who worked with the Consortium on Safe Labor, a group of 19 hospitals that contributed volunteers and data to the study. "And I have to admit, much higher than I was expecting."

Despite the accepted 37-week full-term cutoff, the American College of Obstetricians and Gynecologists officially recommends that babies not be delivered (unless medically necessary) until after 39 weeks. Yet some 9% of all deliveries in the U.S. still occur just shy of 37 weeks, and a recent study found that as many as 23% of late pre-term births — between 34 weeks and 37 weeks — occur for no documented medical reason.

Hibbard says she was surprised by the high rate of health problems she and her team found in this group, since advances in neonatal care have allowed more high-risk babies to survive and eventually thrive after being born too early. But the fact that respiratory distress syndrome continued to occur in premature infants at 40 times the rate in full-term babies highlighted how risky premature delivery can be. At 34 weeks, infants' lungs and respiratory systems are not fully developed, making it difficult from them to survive outside the womb.

With every week of gestation after 34 weeks, however, Hibbard found that rates of complications dropped — by 40 weeks, only 0.3% of babies showed signs of respiratory distress. While 67% of babies required admission to the neonatal intensive care unit at 34 weeks, only 7% of those born at 38 weeks required the same care. Further, 1.5% of babies delivered at 34 weeks developed pneumonia, compared with practically none of those born at 38 weeks. "To be honest, with studies like this, it's hard to justify deliveries before 39 weeks," says Dr. Richard Waldman, president of the American College of Obstetricians and Gynecologists.

Both Hibbard and Waldman stress that there may be valid medical reasons for delivering a baby early — if the mother has pre-eclampsia, the dangerously high blood pressure that can occur during late pregnancy, for example, or if the baby is no longer growing properly in the womb — but that doctors should discourage elective delivery before 39 weeks.

The new study adjusted for many of the major contributors to prematurity, such as the mothers' weight and history of other medical conditions, including pre-eclampsia and diabetes, but the relationship between early delivery and risk of respiratory distress in the babies remained. "I know mothers may request early delivery for a lot of reasons," says Hibbard. "But I hope the obstetrician will pull this study out and say, 'Look, early delivery is not a good idea unless there is really a strong medical indication.'"