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.