Average pregnancy in the U.S. is now 39 weeks

The most common length of pregnancy in the United States is now 39 weeks, a week shorter than the traditional definition of a full-term pregnancy. This shift occurred between 1992 and 2002, according to a new analysis by the March of Dimes published this month in a special supplement of the journal "Seminars in Perinatology."

In 2002, one-quarter of all singleton babies were born full term at 39 weeks. Births at or after 40 weeks decreased by nearly 21 percent. During the decade studied, there was also a 12 percent increase in births occurring between 34 and 36 weeks, referred to as "late preterm births" (sometimes called "near-term births").

"Late preterm infants are a growing concern," said Nancy Green, M.D., medical director of the March of Dimes. "Some babies born just a few weeks early need medical and nursing attention beyond that given to full term newborns. They have a greater likelihood of breathing problems like respiratory distress syndrome (RDS), feeding difficulties, temperature instability (hypothermia), jaundice and reduced brain development than full-term babies."

The March of Dimes analysis suggests that increasing rates of Cesarean section deliveries and induced labor have probably contributed to, but do not completely explain these shifts in deliveries, said Michael Davidoff, Manager of Informatics, Research and Development at the March of Dimes and the paper's lead author.

Clinicians weigh the risk for the mother and the fetus of continuing a medically complicated pregnancy, versus the risks associated with earlier delivery. For some high-risk pregnancies, early delivery may promote better outcomes for both the mother and the baby. The availability of more data on the outcomes of late preterm births will better inform providers and the public about potentially preventable risks. Pregnancies should continue to term if medically and obstetrically advisable, thereby avoiding unnecessary preterm inductions and c-sections.

The March of Dimes study, "Changes in Gestational Age Distribution Among U.S. Singleton Births: Impact on Rates of Late Preterm Birth, 1992 to 2002," was one of the research papers presented at a symposium addressing late preterm birth in July 2005, hosted by the National Institute of Child Health and Human Development of the National Institutes of Health. The scientific meeting focused on definitions, trends and complications faced by late preterm babies. These studies are published in a "Seminars in Perinatology" special supplement on late preterm birth.

The March of Dimes works to improve the health of babies by preventing birth defects, premature birth and infant mortality.