Your odds of having a boy or girl might not be 50/50—here's what affects it
A massive new study from researchers at the Harvard T.H. Chan School of Public Health offers surprising clarity. After analyzing data from more than 146,000 pregnancies, scientists found that birth sex may be influenced by specific factors like mom's age or the sexes of older siblings. The odds, it turns out, might not be 50/50 after all.
The study focused on biological sex assigned at birth, not gender identity, which can develop differently for each person.
For families raising all boys or all girls, this could finally offer an explanation. And if you're hoping your next baby brings a different dynamic, this research offers a little insight—and maybe even a little peace.
What the study found
The research, published in Science Advances, drew on data from the long-running Nurses' Health Study. Rather than assume each baby has an even chance of being a boy or a girl, the researchers looked for patterns across families and found a few compelling ones.
Here's what they found:
Family patterns matter. Families with three or more boys were more likely to have another boy. The same held true for girls.
After three girls, the odds of a fourth girl were about 58%.
After three boys, the odds of a fourth boy were about 61%.
Maternal age may play a small role. Women who began having children after age 28 were slightly more likely to continue having children of the same sex. This may relate to hormonal changes or shifts in vaginal pH.
Genetics might be involved. Two genetic markers showed potential links to sex patterns, though researchers say these findings are early and need more study.
While the study doesn't offer a formula, it suggests birth sex may follow subtle biological rhythms within families.
Related: Why waiting to find out your baby's sex can be (surprise!) awesome
Why some families may be more likely to have boys or girls
At the population level, birth sex tends to balance out, with slightly more boys born worldwide. But zoom in on individual families, and a different picture can emerge.
The researchers describe it like flipping a coin that's not perfectly fair. Some families seem to have a coin that lands on 'boy' more often; others, 'girl.' That tilt, it turns out, may be more biology than luck.
'If you've had two girls or three girls and you're trying for a boy, you should know your odds are not 50-50,' The study's lead researcher Jorge Chavarro told Washington Post, 'You're more likely than not to have another girl.'
The same is true for families with three boys. While the difference is subtle, the large size of the study makes these findings hard to ignore.
Related: Choosing Your Baby's Sex.
What doesn't make a difference
Just as important, the study helped clarify what doesn't appear to influence a baby's sex.
Researchers found no meaningful connection between birth sex and:
Race or ethnicity
Blood type
Body mass index (BMI)
Height
Natural hair color
Lifestyle habits, certain foods, or timing methods weren't included in this study and remain unproven.
Since the study only included maternal data, paternal influence is still unknown.
Related: Gender disappointment is real—and if you feel it, you're not alone
What this means for families
When you're raising three boys or three girls, people tend to notice. The questions start early—'Are you going to try again for a girl?' or 'Hoping for a boy this time?' and while often well-meaning, they can be exhausting.
The truth is, many families didn't plan to end up with all boys or all girls. And according to this research, these streaks may reflect a biological pattern.
That can bring a sense of relief. It shifts the narrative from personal choice or superstition to something more grounded: biology doing what it's wired to do.
TikToks from #boymom and #girlmom creators show just how many families are joyfully navigating this path. And while those hashtags celebrate shared experience, it's worth remembering: every child is unique—personality isn't shaped by birth sex alone.
Related: 'I dreamed of a daughter': This mom's emotional story of gender disappointment is resonating with parents
Hoping for a mix?
If you're hoping your next baby brings variety, you're not alone. Many parents do. This study doesn't offer a strategy, but it does offer perspective.
The pattern in your family might gently continue. It's not a guarantee, but it's worth knowing. And whatever feelings come up—joy, longing, even disappointment—can all exist alongside deep love for the child you're welcoming.
Related: To the mama preparing her heart for baby #2: You are ready
The bottom line
Science is still uncovering how families take shape. But this research offers reassurance for anyone who's noticed a pattern and wondered if it meant something more.
Whether you're raising a girl gang, a pack of boys, or something in between, there's no 'right' mix. Each child brings their own kind of balance. And every family, in its own way, is already whole.
Source:
Science Advances. 2025. 'Is sex at birth a biological coin toss? Insights from a longitudinal and GWAS analysis'
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