Weight Gain During Pregnancy Is Especially Good for Boys

The less weight a woman gains during pregnancy, the less likely she is to have a boy, an analysis of data on more than 68 million births over 23 years has found.

Over all, 51 percent of babies born are boys. But Kristen J. Navara, an endocrinologist in the Poultry Science Department at the University of Georgia, found that the amount of pregnancy weight gained and the proportion of males born go up together. When mothers gain 20 pounds, roughly 49 percent of babies born are boys. At 40 pounds gained, about 52.5 percent are boys; and at 60 pounds, about 54 percent are boys. Above 60 pounds, the correlation disappears. 

The reasons are unclear, but Dr. Navara said that male embryos and fetuses have higher metabolic rates and may require more nourishment to develop successfully. Low maternal weight gain may therefore be more detrimental to boys than girls.

“More than 13 million women in our study gained fewer than 20 pounds during gestation — this comes out to approximately 525,200 ‘missing’ males,” Dr. Navara said.

“For women who are older,” she added, “we do tests around 11 weeks to find chromosomal problems and we incidentally discover the sex. Maybe it’s worth doing that for everyone so that we can optimize the conditions necessary to survival.”