Scientists may have stumbled upon a new clue to one of life's great mysteries: boy or girl? If you're heavier at the start of your second pregnancy than you were at the start of your first, there's a slightly higher chance you'll conceive a boy this time around, according to a study of more than 220,000 Swedish women.
The researchers had set out to test whether the rise in obesity was behind an increasingly smaller male-to-female ratio, but found the opposite effect among the study population.
There's no clear explanation, says lead researcher Eduardo Villamor, D.P.H., M.D., of the Harvard School of Public Health; one possibility is that environmental contaminants—some of which have been linked to an increase in male births—may build up in fatty tissue.
Villamor hopes his research won't encourage boy-crazy women to pile on the pre-pregnancy pounds, as being overweight "poses very dangerous risks for the mother and the baby."