Infants born at 37 or 38 weeks are twice as likely to die in first year as those born later, study finds

Birth 37 weeks after conception is normally considered a full-term pregnancy, but new research indicates that delaying delivery for another week or two can halve an infant’s risk of dying in his or her first year, researchers say.

“Infants born at 37 weeks or greater are considered term and have traditionally been considered as a uniform group,” said Dr. Uma M. Reddy of the Eunice Kennedy Shriver National Institute of Child Health and Human Development, lead author of a new study appearing in the journal Obstetrics & Gynecology. “Our study shows that there are often marked differences in the health of infants born in the early part of this period and those born in the later part.”

Reddy and her colleagues from the National Institute of Child Health and Human Development and the March of Dimes studied data on the birth of more than 46 million infants born in the United States between 1995 and 2006. The data, from the Centers for Disease Control and Prevention’s National Center for Health Statistics, linked all infant death certificates in a given year to their corresponding birth certificates in the same or preceding year.

The researchers found that, between 1995 and 2006, the proportion of infants born at 37 or 38 weeks -- a period called early term -- increased from 22% to 29%, while the proportion of births occurring at 40 weeks or greater -- called full-term births -- fell from 60% to 54%. Infants born at 37 weeks were twice as likely to die before their first birthday (3.9 deaths per 1,000 births) as those born at 40 weeks (1.9 deaths per 1,000 births). The most common causes of death included birth defects, sudden infant death syndrome, lack of oxygen in the womb or during birth, and accidents.

“It is clear that, regardless of race or ethnicity, every additional week of pregnancy is critical to a baby’s health,” said Dr. Alan R. Fleischman, medical director of the March of Dimes. While there are times when medical reasons require a baby to be delivered early, an early elective delivery is harmful to a baby and should never be scheduled before 39 or 40 weeks of pregnancy, he added.