Neonates who are delivered at 36-38 weeks' gestation after fetal lung maturity is confirmed have nearly double the risk of adverse outcomes, compared with neonates delivered at 39 or 40 weeks, a large retrospective cohort study has shown.
The mean birth weight in 459 neonates with confirmed lung maturity who were delivered at 36-38 weeks' gestation was 3,017 g, compared with 3,362 g in 13,339 neonates delivered at 39-40 weeks. The risk of a composite outcome including death, adverse respiratory outcomes, hypoglycemia, treated hyperbilirubinemia, generalized seizures, necrotizing enterocolitis, hypoxic ischemic encephalopathy, periventricular leukomalacia, and suspected or proven sepsis was 6.1% in those in the 36- to 38-week group, compared with 2.5% in the 39- to 40-week group, Dr. Elizabeth Bates of the University of Alabama at Birmingham and her colleagues reported.
Early delivery remained a significant risk factor for the composite outcome after investigators adjusted for maternal age, ethnicity, parity, neonatal sex, intended mode of delivery, and any medical complication – including diabetes and hypertension. Early delivery was also a significant risk factor for several individual outcomes, including respiratory distress syndrome (adjusted odds ratio, 7.6); treated hyperbilirubinemia (AOR, 11.2); and hypoglycemia (AOR, 5.8), the investigators found (Obstet. Gynecol. 2010; 116:1288-95).
The incidence of the primary composite outcome generally decreased with increasing gestational age, they noted (9.2% incidence at 36 weeks, 3.2% at 37 weeks, 5.2% at 38 weeks, and 2.5% at 39-40 weeks).
Patients included in the study were women with a singleton pregnancy receiving prenatal care and giving birth at a single center from January 1999 to December 2008. Among those who were delivered at 36-38 weeks following documentation of fetal lung maturity, 42.5% had completed 36 weeks, 40.7% had completed 37 weeks, and only 16.8% had completed 38 weeks. Of those who were delivered at 38-40 weeks, 56.2% had completed 39 weeks, and 43.8% had completed 40 weeks. The mean gestational age was 37.1 weeks in those delivered at 36-38 weeks and was 39.8 weeks in those who delivered at 38-40 weeks.
The study findings are concerning, because fetal lung maturity is known to reduce the risk of respiratory morbidity, and confirmation of fetal lung maturity is “a recognized exception to longstanding recommendations against elective delivery before 39 weeks' gestation,” Dr. Bates and her associates noted.
Also, despite existing recommendations to the contrary, one-third of elective cesarean deliveries in one large study were performed before 39 weeks, they said.
Taken together, the findings in the current study “are consistent with relative immaturity at 36-38 weeks (regardless of lung maturity), compared with 39-40 weeks, and lower threshold for admission to the NICU and for invasive sepsis work-ups (suspected sepsis),” the investigators wrote.
They added that the findings should be considered in light of the study's limitations – including the retrospective study design and the related possibility of confounding, and the fact that the study does not fully address the risk of stillbirth associated with either delivery strategy studied. Nonetheless, they concluded that the findings suggest that “in the absence of ongoing concern about fetal death or maternal well-being if the pregnancy continued, delivery should be delayed until 39 weeks.”
The findings also suggest that purely elective fetal lung maturity testing and early delivery should be avoided, Dr. Bates and her associates noted.
One of the study authors, Dr. Alan T. N. Tita, was a Women's Reproductive Health Research Advanced Scholar at the University of Alabama at Birmingham at the time of the study and received funding from the National Institute of Child Health and Human Development. No relevant financial disclosures were reported by the other authors.
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Reconsider What Constitutes Term Birth
“This paper further advances our understanding of the optimal conditions for delivering a baby free of major health complications. Specifically, it confirms findings from a number of previous studies suggesting that babies born before 39 weeks' gestation have a significantly increased risk of complications, including respiratory distress syndrome, even if their lungs appear to be fully mature via biomarker testing,” Dr. E. Albert Reece said in an interview.
More importantly, he said, the study suggests that current definitions of what constitute a term birth may need to be reconsidered.
“Indeed, currently babies born between 37 and 42 completed weeks of pregnancy are considered full term, whereas babies born before 37 weeks of pregnancy are completed are considered preterm. However, if the results of this study are to be believed, they suggest that babies born before 39 weeks of gestation might be considered preterm as well.”