U.S. newborns are arriving a little smaller, says puzzling new Harvard research that can't explain why. Fatter mothers tend to produce heavier babies, and obesity is soaring. Yet the study of nearly 37 million births shows newborns were a bit lighter in 2005 than in 1990, ending a half-century of rising birth weights.
The change isn't big: The average birth weight of full-term babies is just under 7 1/2 pounds, a drop of about 1.8 ounces, researchers reported Thursday in the journal Obstetrics & Gynecology.
That's surprising considering doctor warnings about 9-pound, or bigger, babies. So the researchers double-checked.
The proportion born large for their gestational age dropped about 2 percent, which is good.
"What physicians are responding to is that the bigger babies are getting bigger," said lead researcher Dr. Emily Oken of Harvard Pilgrim Health Care. Plus, "babies are still bigger than they were 30, 40, 50 years ago. It's just the trend seems to have flattened or reversed itself."
That's particularly true for women at lowest risk for too-small babies: White, well-educated, married non-smokers who got early prenatal care. Still, their babies, on average, weighed 2.8 ounces less over the study period.
Babies born too big are at increased risk of obesity and diabetes later in life. On the other hand, babies born too small may require intensive care right away and also be at risk for later chronic diseases. The proportion of babies small for their gestational age did increase slightly, by 1 percent, Oken said.
Moreover, babies' length at birth suggests even full-term pregnancies are 2.5 days shorter than they used to be. That can't account for all the weight change, and Oken couldn't find a full explanation from the birth certificates she studied.
Oken excluded premature babies, as well as twins or other multiples, from her study. (Obese mothers also are at higher risk of having a preemie.) Yes, there are more scheduled cesarean sections or induced labors now, but her analysis concluded that wasn't to blame.
But that's not clear as induction often isn't listed on birth certificates, and the study found a drop in babies born at 40 or 41 weeks gestation, noted Dr. Joann Petrini, an adviser to the March of Dimes.
Regardless, it's too soon know what this drop means for babies but "we should keep vigilant" about any decline among the smallest newborns, Petrini said.
The study uses the most recent data available from official U.S. birth certificates.