After two girls or two boys, is the third child really 75% likely to be the same sex? Le Paternel looked at French census data to check the claim. Our reported results were close to 50/50—and a newer American study does not settle the question on its own.
A girl or a boy: a coin toss?
It is an appealing shorthand. During a first pregnancy, finding out whether everything is going well usually matters rather more than completing a particular family line-up. By the second child, however, well-meaning people may start talking as if you were collecting a set: “Ah, you already have that one.”
The French expression le choix du roi, meaning a boy and a girl, has its own historical baggage: a son to carry on the name and a daughter to arrange a useful marriage. Fortunately, children are not trading cards or diplomatic agreements.
By the third pregnancy, the unsolicited statistics often get more confident.
“After two children of the same sex, the third has a 75% chance of being the same sex”
That was the claim we wanted to check. Where did the number come from? We had not found a convincing source for it.
So we turned to the French national statistics office’s 2014 census data. Martial, accustomed to working with large datasets in mobile gaming, helped extract the figures for the original article. Martial has two daughters. Coincidence? We will resist calculating that one.
What our French household analysis reported
The original analysis considered households with three children whose first two children were the same sex. The reported percentages were rounded:
| Children, in order | Share |
|---|---|
| Girl, girl, boy | 29% |
| Girl, girl, girl | 28% |
| Boy, boy, girl | 21% |
| Boy, boy, boy | 22% |
Adding the two same-sex groups gives 50%; adding the groups with a different-sex third child also gives 50%. Calculating separately from these rounded figures gives approximately 49% girls after two girls and 51% boys after two boys.
In our reported extraction of the 2014 French census, the results therefore did not support the 75% claim. This is a description of that analysis, not a guarantee for an individual pregnancy.
There is a limitation worth making explicit: a household census is not automatically a complete record of every biological birth. The original extraction’s exact filters, raw counts and uncertainty estimates need to be recovered before we can independently reproduce the calculation. The rounded chart alone cannot establish an exact, universal 50/50 rule.
Update: what about the 2025 American study?
A 2025 study in Science Advances analysed 146,064 pregnancies among 58,007 US women and reported more clustering of children’s sex within families than a simple independent coin-toss model would predict.
But Judith Lok and Mireille Schnitzer’s methodological critique, available on arXiv argues that family-size selection and parents’ preference for having children of both sexes can generate such patterns even with independent births. The study’s authors attempted adjustments, including removing the last birth; the critique challenges the resulting inference.
The important distinction is between observing the make-up of completed families and predicting the next birth after two children. They are not automatically the same calculation. The debate is a reason to describe the evidence carefully, not to replace the French result with a new certainty.
Why does the rumour travel so well?
The original article offered a theory: perhaps people repeat the figure to discourage parents from having a third child solely in pursuit of a boy or a girl. That was speculation about the rumour, not a demonstrated explanation of medical professionals’ behaviour.
The underlying advice does not need a made-up percentage. Wanting another child and wanting a particular outcome are different things. A third child is a whole person, not a missing item in the collection.
Can a diet or an app tip the odds?
The original French article also discussed MyBubelly, a commercial service combining cycle tracking, dietary advice and supplements, alongside claims about pH. That historical coverage should not be read as proof that these methods allow parents to choose a child’s sex.
This international edition does not repeat the old price or present the advertised mechanism as established science. Neither the household figures above nor the 2025 debate validates a commercial sex-selection programme. Before following a restrictive diet or taking supplements while trying to conceive, discuss it with an appropriate healthcare professional.
The useful answer remains modest: our French analysis did not find the advertised 75%, and the newer study needs to be read alongside its methodological criticism. Anyone promising you a sure route to completing the set deserves a few questions.
Original photograph: Kelly Sikkema.
