Mayan gender calendar & calculator

How the Mayan gender predictor works

The rule needs two numbers:

• The mother's age at conception, in completed years, the way you'd normally give your age.
• The month of conception as a number, from January = 1 to December = 12.

That's the whole method. Every popular version we checked gives the same direction: same parity is a girl, mixed parity is a boy. We found this form on five separate pregnancy and testing sites (listed in the sources), and none of them flipped it. One site notes that some calculators use the year of conception instead of the month. We use the month version, which is the one almost everyone publishes.

Working out age and month

Age. The age that counts is the mother's age on the day of conception, not at the due date or today. If conception falls in her birthday month, the exact day matters: conceiving on the 10th with a birthday on the 20th means she was still the younger age, and that alone flips the prediction. The calculator handles this for you.

Due date mode. Most people don't know the exact day of conception. A due date is usually set 280 days after the first day of the last period, and ovulation typically happens about two weeks after that period starts. So conception is roughly the due date minus 266 days. It's an estimate: ovulation can come earlier or later, so if the estimated date lands near the start or end of a month, the month number, and with it the prediction, could easily be the other one. The calculator shows the estimated date so you can see how close to the edge it is.

Is it really Mayan?

We found no evidence that it is. The ancient Maya kept time with their own calendars, including a 260-day ritual cycle and a 365-day year, not with months numbered January to December. The rule depends entirely on those modern month numbers. We could not find it in any historical source, and the pages we read that discuss its origin don't cite one. At least one of the sites that publishes the calculator says its Mayan origin may be "more fiction than fact".

So the honest description is modern folklore with an ancient-sounding name. That doesn't stop it being fun. It just means the name tells you nothing about whether it works.

Does it work?

We found no study that has tested the Mayan rule. There's also no reason to expect it to work: whether a baby is a boy or a girl is set by the sperm's X or Y chromosome at fertilization, and the parity of the mother's age has nothing to do with that.

Any rule that isn't linked to the baby's sex will be right about half the time, simply because roughly half of babies are girls. In Danish national data, for example, there were 1.04 boys born for every girl. That built-in 50% is why the calendar seems to work for so many people: every correct guess gets remembered and shared.

Mayan vs Chinese gender calendar

The two are often mentioned together, and they share the same idea: the mother's age plus the month of conception. The difference is that one of them has been properly tested.

The Chinese calendar study used every single-baby birth in Sweden from 1973 to 2006. Researchers worked out each mother's lunar age and lunar month of conception and applied a published version of the chart. Agreement between the predicted and actual sex was essentially zero, and it didn't improve by maternal age, education or number of previous children. Their advice was not to paint the nursery based on it. See our Chinese gender calendar to try it, and how accurate it is for the full study.

If you try both with the same dates, they'll often disagree. That's what you'd expect from two guesses that are each unrelated to the baby.

More ways to guess, for fun

If you enjoy this, take our boy or girl quiz, which runs through the classic old wives' tales and then shows the evidence for each one, or read the old wives' tales guide. For methods that look at the baby itself, see nub theory at 11–13 weeks. Your anatomy scan, or a blood test offered by your doctor, is what actually tells you.

FAQ