Want a Mix of Boys and Girls? Timing Might Matter, Study Says

When it comes to predicting whether your next baby will be a boy or a girl, plenty of theories float around. Maybe it’s how you’re carrying, your morning sickness, or even your cravings. But while most of these signs fall squarely in old wives’ tale territory, the truth has generally held steady that it’s a 50/50 chance. That is, until now.

A new study published in Science Advances suggests the scales may be tipped a little in one direction or the other in certain families. Researchers analyzed data from over 58,000 families with at least two children to find out whether the sex of a child truly is random. To avoid skewing the results, they excluded each family’s most recent child to account for “sex-based stopping behavior,” or the idea that parents may stop having kids once they get a boy or a girl.

After reviewing the data, researchers found that families with three boys had a 61% chance of having a fourth boy. And families with three girls had a 58% chance of having a fourth girl. What’s more, moms who waited until later in life to have their first pregnancy were more likely to have all girls or all boys.

The cause for the link between later-in-life pregnancy and single-sex families can’t be pinpointed, but researchers suggest that biological changes with age—like a shorter follicular phase and lower vaginal pH—may influence which type of sperm (X or Y chromosome) has a better chance at fertilizing the egg. A shorter follicular phase may favor Y-bearing sperm (boys), while a more acidic environment may support X-bearing sperm (girls).

Of course, experts emphasize that more research is needed before any solid conclusions can be drawn. The study’s authors note that many other genetic or environmental factors could be involved, and that the effect, while statistically significant, may not be strong enough to make individual predictions.

Still, the findings offer an interesting glimpse into how timing might play a subtle role. For families hoping for a mix of boys and girls, getting started earlier may slightly increase the odds, but that doesn’t mean those starting later are out of the running. Every family’s journey is unique, and no matter when you begin or what the sex may be, each child is no doubt a miracle worth celebrating.