Ever feel like your family is stuck on repeat when it comes to your baby’s gender? If you’re the proud parent of three (or more!) kids all on the same team, you’re not alone — and you’re not imagining things. A fascinating new study from Harvard University looked at the birth records of over 58,000 moms and found something surprising. When it comes to larger families, the gender of your kids might not be a simple coin toss after all.
Instead, scientists say it’s more like a weighted coin toss. Meaning, your family might naturally lean toward having more boys or more girls. So if your household feels like a mini sorority or a nonstop superhero squad, there actually may be some science behind it.
All Boys or All Girls? Science Says That Might Not Be Just Luck
If you’ve ever asked, How did we end up with all girls? you’re not alone. There’s a lot to know about baby’s gender.
“We’re seeing a lot of families that seem to be all-in on one gender,” says Siwen Wang, a doctoral student at Harvard and lead author of the new study. The all boys or all girls question got Wang and her team wondering: Is it just random chance, or is something else going on here?
So, they dove into decades of data from the Nurses’ Health Study — tracking more than 146,000 pregnancies across the U.S. — and the results were eye-opening. Turns out, the so-called 50/50 boy-girl odds might not be so even after all, especially in bigger families.
Here’s what they found:
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If your first two kiddos are girls, you’re more likely to have another girl next.
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Same goes for boys — two boys? The third is more likely to be a boy, too.
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In fact, families with three boys had a 61 percent chance of adding a fourth boy. For three girls, it was 58 percent likely the next would be another girl.
And here’s another twist: Mom’s age when she starts having kids may play a role.
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Moms who had their first child before age 23 had around a 40 percent chance of having all boys or all girls.
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Start after age 28? That chance rises to about 50/50 — a small difference, but enough to get scientists curious.
What Science Still Doesn’t Know About Baby’s Gender
Researchers wanted to get to the bottom of why some families seem to have all boys or all girls — and they were pretty picky about who they included in the study. They left out families with only one child, pregnancies that ended in miscarriage or stillbirth, twins, and anyone who had infertility treatments. They also didn’t count the last baby in a family, just in case parents stopped once they hit their “perfect mix.”
Now, while the study uncovered some interesting patterns, there’s still a lot that’s a mystery. One idea? As women get older, their cycles get shorter, which might influence which kind of sperm (X for girls or Y for boys) gets the upper hand. Also, changes in a female’s acidity could possibly play a role in favoring one over the other.
“We don’t know the dad’s age or genetics — and that matters for baby-making,” says Wang. In fact, other scientists agree that dad might be half the story here.
The bottom line? Science is starting to unlock why some families are “all boys” or “all girls” — but it’s a complicated puzzle that includes moms, dads, biology, timing … and maybe just a sprinkle of mystery.
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