As soon as Keiko Ozaki found out she was pregnant with her second baby, she went on a diet.

Ozaki, who teaches jazz dancing in Osaka and weighed 51 kg, said she was chastised by her doctor for gaining weight too quickly during her first pregnancy. With her second baby, Ozaki, 30, avoided rice and skipped meals before her monthly checkups, adding 7.8 kg over nine months — 4.7 kg less than U.S. women gain on average.

She managed to avoid her doctor's rebukes, but her baby boy was 300 grams lighter than the national average of 3 kg. As Japan's birth weights fall for a third decade, scientists say advice that pregnant women receive may be contributing to the highest rate of low-birth-weight babies in the developed world. More critically, it may be setting their infants up for diabetes and heart disease later in life.