Weight Gain in Pregnancy Linked to Sex of Baby

The less weight a woman gains during pregnancy, the less likely she is to have a boy, an analysis of CDC data found.

The reasons behind the correlation of a mother’s gestational weight gain and the sex of her child are unclear. But Kristen J. Navara, an endocrinologist in the Poultry Science Department at the University of Georgia, found after sorting through more than 68 million births, that the amount of weight gained during pregnancy and the proportion of males born are correlated.

Key Points:

- Gaining less weight during pregnancy correlates with a decreased likelihood of giving birth to a boy, an analysis of 68 million births found.

- However, this correlation disappears when gestational weight gain exceeds 60 lb.

When mothers gained 20 lb, roughly 49% of babies born were boys. But when mothers gained 40 lb, the likelihood of having a boy increased, with about 52.5% of babies born being males. And at 60 lb gained, about 54% of babies were boys.

The relationship between a mother’s weight gain and the likelihood of having a boy disappears if the woman gains more than 60 lb, however.

Navara said the phenomenon was something she suspected given that previous studies have shown how severe food restriction will influence the sex ratios.

“I hypothesized that the ratio of male to female babies born should vary with the amount of weight gained during gestation,” she wrote. “I predicted that women who gain low amounts of weight during gestation should produce significantly more females, and that, if gestational weight gain directly influences sex ratios, fetal losses would be more likely to be male when women gain inadequate amounts of weight during pregnancy.”

In analyzing the data, she looked at vital statistic information collected over 23 years and included looking at the sex of fetal losses, finding that fetuses miscarried prior to 6 months' gestation were significantly more likely to be male if the mother had gained low amounts of weight during the pregnancy. The finding suggests that low caloric intake during early fetal development may be a factor in the resulting miscarriage, she said.

The study was published in PLOS One.