Second Baby Gender Prediction: Does Your First Child Give a Clue?

In July 2025, researchers at the Harvard T.H. Chan School of Public Health and Brigham and Women's Hospital published “Is sex at birth a biological coin toss?” in Science Advances (Wang and colleagues). They studied 58,007 American women who had at least two single births, 146,064 pregnancies in all, between 1956 and 2015.

If every birth were an independent fair coin toss, the number of boys and girls in families would follow what statisticians call a binomial distribution. It did not. The numbers fitted a beta-binomial distribution better, which the authors describe as each family possibly having “a unique probability of male or female births, akin to a weighted coin toss.” In other words, more families end up with all boys or all girls than pure chance would produce.

The study also looked for reasons. After setting aside each woman's last birth (to remove the effect of parents who keep trying until they get the other sex), an older age at first birth was linked to higher odds of having children of only one sex. A genetic analysis found variants in the mothers near two genes, NSUN6 linked to having only girls and TSHZ1 linked to having only boys.

It means your first child is not completely irrelevant, which is a genuine change from what most of us were told. It does not give you a usable prediction. The abstract describes a pattern across tens of thousands of families; it does not publish an odds table you could apply to yours, and a single child is very little evidence about which way your own coin is weighted.

Two cautions are worth adding. First, families who stop after getting the sex they wanted make all-boy and all-girl families look more common than they are; the authors handled that by excluding last births, which is exactly why their result is worth taking seriously. Second, a weighted coin is still a coin. Even in a family that leans one way, the next baby can be either sex, and plenty of three-boy families have a daughter fourth.

The sex itself is still set at fertilization, by whether the sperm carries an X or a Y chromosome, as MedlinePlus Genetics explains. The Harvard work hints that a mother's biology may tilt those odds slightly within a family; it does not suggest that anything you do in the bedroom, the kitchen or the calendar can.

The Chinese chart has no column for birth order and no memory of your first child. It uses the same two inputs every time: your lunar age at conception and the lunar month of conception. So for a second baby you simply recalculate both. Your lunar age will be higher, which moves you to a different row, and that is why the chart often disagrees with what it told you last time.

We measured how much. On the chart this site uses, we compared every row with the row one and two lunar years below it, month by month:

Counted from the chart in this site's calculator (lunar ages 18 to 45), October 2026.

For example, in lunar month 3 the chart says girl at lunar age 30 and boy at lunar age 32. Two years, same month, opposite answer. To recompute your own, use the lunar age calculator and our guide to the lunar month of conception, or put both dates into the Chinese gender calendar calculator.

And remember the chart's record. In the largest test, on 2,840,755 Swedish births, it matched the baby's sex no better than chance, and that was true whether or not the mother already had children (Villamor et al., 2010). Parity was one of the factors those researchers checked.

Some parents hoping for “one of each” use the chart to pick a conception month. If you want to try it for fun, the first job is to find your row for the year you plan to conceive, because it will not be the row you used last time. Here is what five common rows look like on this site's chart:

From the chart in this site's calculator.

Then convert those lunar months into real dates for the year you are planning, using our 2027 or 2028 chart pages, and remember that the real-world odds do not move. Our guide to when to conceive with the Chinese calendar covers the planning ritual in more detail, and the ovulation baby gender calendar explains why timing within the month does not change the sex either.

Take a mother born on September 12, 1994. Chinese New Year 1994 was on February 10, so her lunar birth year is 1994. Her first baby was conceived on May 1, 2024, which is in lunar month 3 of 2024 (April 9 to May 7, 2024). Her lunar age then was 2024 − 1994 + 1 = 31, and the chart, at age 31 in month 3, says boy.

Two years later she conceives again, on May 1, 2026. That date is also in lunar month 3 (April 17 to May 16, 2026). But her lunar age is now 2026 − 1994 + 1 = 33, and at age 33 in month 3 the chart says girl. Same date, same month number, opposite answer, purely because she moved two rows down the chart.

This is the most common reason parents tell us the chart “changed its mind” between children. It did not change; the mother's row did. Month dates are from the Hong Kong Observatory tables for 2024 and 2026.

It is tempting to read the 2025 study as “two boys means a third boy is likely”. The published abstract does not say that, and nobody should turn it into a calculator. Three gaps are worth keeping in mind. The women studied were all in the United States, so the pattern may not hold everywhere. The genetic findings point to variants that may matter; they are not a test you can take. And a family-level tilt, however real, says little about any one pregnancy.

What the study does change is the conversation. When a relative insists that your family “only makes boys”, that is no longer pure superstition: families can lean. It is still a lean, not a rule.

Second pregnancies come with a special category of folklore: “this one feels different, so it must be the other sex.” It is worth knowing that a Johns Hopkins study of 104 pregnant women found that comparisons with previous pregnancies were not systematically related to the baby's sex, and neither were belly shape or morning sickness (Perry, DiPietro and Costigan, 1999). Every pregnancy feels different; that is not a signal.

If you want something fun to do while you wait, the Chinese gender reveal calendar page has ideas that work for siblings, and the early pregnancy gender prediction guide covers the medical tests that actually answer the question.