Can the sexes of your partner's siblings predict if you'll have a boy or girl?

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posted in Pregnancy

My mom is the eldest of six. One of our favorite family stories to tell is about the time my uncle (who was welcomed into the world by four older sisters) burst into tears upon hearing my grandparents’ last baby was another girl.

The poor guy just wanted at least one brother…and then he grew up and had two daughters of his own.

Coincidence? Perhaps not.

A recent study by Newcastle University involving thousands of families has shown that men inherit a tendency to have more sons or more daughters from their parents, in particular their fathers.

The research looked at 927 family trees containing information on 556,387 people from North America and Europe going back to 1600. To put it simply, it was found that men with many brothers are more likely to have sons and men with many sisters are more likely to have daughters.

As Dr. Claudia explained it for BabyCentre UK:

Why? Well it seems that men have an (as yet undiscovered) gene that codes for whether they have more Y sperm (male baby-making sperm) than X sperm (female baby-making sperm).

Men whose fathers have the gene coding for more male baby-making sperm are likely to pass that gene on to their sons and so their sons are more likely to have boys than girls. And vice versa.

So while my grandparents were fortunate to have one boy along with their five daughters, it shouldn’t have been any surprise he went on to have girls.

It’s easy to see this working with my mom’s siblings but when it comes to my own family I’m less sure. My father had four brothers and one sister, so you might expect my parents would have had sons — but I (female, to be clear) popped out two years after my one and only brother.

And does it work as much of a predictor with mixed-sex siblings? My husband had one brother and one sister. We have two sons, so maybe? But then again, his brother has one son and two daughters.

Of course the findings are about the general population and don’t apply to every circumstance, and it seems easier to see a pattern in larger families. Still, it’s fun to take a peek and try to see if this rings true anywhere in your family tree.

Images via UnSplash/Edward Cisneros, designhorf, Matheus Ferrero

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