Throughout the past few decades, birth weights have risen. Babies born in 2005 are weighing 1.8 ounces less than babies born 15 years earlier.
Researchers are startled because women have become larger, they're smoking less and are older when they have children -- all factors that should contribute to bigger babies.
They think it could be environmental factors or mother's diet or physical activity.
The study is in the Journal Obstetrics and Gynecology.