🌅 The Real Odds of Having a Boy or Girl

SOURCE

Wang et al. (2025)

WHAT TO KNOW

• A new study by researchers at the Harvard T.H. Chan School of Public Health examined 146,000 pregnancies in the U.S. from 1956 to 2015, finding the odds of having a boy or girl are more akin to a weighted coin toss than a 50-50 chance. More specifically, the team found a baby’s sex at birth is associated with maternal age, certain genes, and the sexes of older siblings.

WHY IT MATTERS

• The researchers found that in families with at least three children of the same sex, the next baby’s sex tended to be the same. In families with three female children, the odds of having another female was 58%; in families with three males, the odds of having another male was 61%.

CONNECT THE DOTS

• Maternal age also influenced a child’s birth sex: women who started having children after age 28 were slightly more likely to have all boys or all girls. The team also discovered two genes linked with having children of only one sex, though more work is needed to understand the association and whether other factors, like lifestyle, nutrition, and exposure to environmental chemicals, play a role in sex at birth.