WASHINGTON – 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.

“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.