Medical disclaimer. This article is general information, not medical advice. Folk gender-prediction methods are entertainment and nothing more. For anything about your baby's health, development, or sex, talk to your midwife or doctor.
The Mayan gender predictor is a single line of arithmetic. Take the mother's age at conception and the number of the month she conceived, then check whether those two numbers are both even, both odd, or one of each. Both the same means girl. One of each means boy. That is the entire method — no chart, no lookup table, nothing measured about the pregnancy itself.
It takes five seconds, it is fun, and it is right about half the time — which is exactly what a coin flip with extra steps returns.
The rule, spelled out
Two inputs, two questions, one answer.
- Mother's age at conception. Not her age today — her age on the day the baby was conceived.
- The conception month, as a number. January is 1, December is 12.
Then compare the two for parity, the maths word for "odd or even":
| Conception month even | Conception month odd | |
|---|---|---|
| Mother's age even | Girl | Boy |
| Mother's age odd | Boy | Girl |
A worked example: a mother who is 29 at conception (odd), conceiving in June (month 6, even). Odd and even do not match, so the method says boy.
Change one input by one step and the answer flips. Conceive in July instead of June and it says girl. Have a birthday the week before conception and it says girl. Nothing about the pregnancy changed — only the labels on two integers did.
Where the "Mayan" part comes from
Short version: not from the Maya.
Classic Maya civilisation kept famously sophisticated calendars — the Tzolk'in, the Haab', the Long Count — and used them to time ritual, agriculture, and dynastic record-keeping. What nobody repeating this rule seems able to produce is a source for it: no published codex, inscription or calendrical table has surfaced that assigns a baby's sex from the mother's age and the month of conception. It reads as folklore wearing an ancient name, in the same family as the ring-on-a-string test and reading the shape of a bump.
The framing is doing real work, though: "Mayan" makes a coin flip feel like inherited knowledge. Enjoy that part. Just do not paint a nursery on it.
Is the Mayan gender predictor accurate?
Any rule that always answers "boy" or "girl" is right roughly half the time, because roughly half of babies are boys. The interesting question is whether a method beats that baseline — and for the closest method anyone has actually tested at scale, the answer is no.
The Chinese lunar calendar chart works the same way in spirit: a mother's age plus a conception month, resolved to a boy/girl cell. A 2023 paper in Actual Gynecology and Obstetrics checked that chart against 17,580 births and reported a kappa of 0.002 (p = 0.797) — statistician for "nothing here." The same paper cites a far larger analysis, by Villamor and colleagues in 2010, involving 2,840,755 patients between 1973 and 2006, which concluded that the "Chinese calendar is no better than flipping a coin" in predicting gender.
The Mayan rule has never been tested that thoroughly, and it does not need to be. It uses less information than the Chinese chart, none of it biological — there is no mechanism by which the parity of a number could influence which sperm arrives first.
So why does everyone know someone it "worked" for? Because half of all guesses land. The hits get retold at baby showers; the misses evaporate. Add a 50/50 rule to selective memory and you have a legend.
Mayan vs the Chinese gender chart
They look like cousins, and they are — but the machinery differs.
- Mayan: pure parity. Two numbers, one odd/even comparison, no table involved.
- Chinese: a genuine lookup. You convert the mother's birth date and the conception date into the Chinese calendar, work out her nominal age (the traditional count that starts at one), find the lunar month of conception, and read the cell where they meet. Traditional charts only cover nominal ages 18 to 45 and months 1 to 12, so plenty of real pregnancies fall off the chart entirely.
The Chinese version takes more work and carries far more history. Neither performs better than chance. If you want to try it, there is a Chinese gender predictor here too.
What actually decides boy or girl
The real mechanism is settled at fertilisation, months before anyone counts a month.
A mother has two X chromosomes, so the egg always carries an X; a father has one X and one Y, so the sperm carries one or the other and sets the pairing: XX or XY. From there, according to MedlinePlus Genetics, the SRY gene on the Y chromosome makes a protein that "starts processes that cause a fetus to develop male gonads (testes) and prevent the development of female reproductive structures." Without a working SRY protein, a fetus with an X and a Y develops a uterus and fallopian tubes instead — which is why sex development is better described as a genetic switch than as a chromosome count.
One more distinction worth keeping straight: these tools predict sex assigned at birth, not gender. Gender is something your child will tell you about later, in their own time.
For the real answer, the NHS says you may be asked at the 20-week screening scan — offered between 18 and 21 weeks — whether you would like to know your baby's sex. It also says finding out is not part of the national screening programme, that some hospitals have a policy of not telling patients, and that it is not possible for the sonographer to be 100% certain: if your baby is lying in an awkward position, it may be difficult or impossible to tell.
Curious about the dating maths behind all of this? How due date calculators work walks through how conception dates and due dates line up, and the science behind inherited traits covers what genetics can and cannot predict.
How ProbaBaby helps
In the ProbaBaby app, the Mayan Gender Predictor asks for two things: the mother's birth date and the conception date, both as real date pickers. It measures her age to the actual conception date rather than assuming the current year, takes the month from that same date, then compares the two for parity — matching parity gives girl, mismatched gives boy. The result is drawn as a two-by-two matrix with your quadrant lit up, so you can see the rule instead of taking the answer on faith, and a live preview recomputes the parity before the reveal. The screen carries the app's disclaimer verbatim: "This is for entertainment only and has no scientific basis."
The app's Chinese Gender Predictor is a different animal. It converts both dates with the Chinese calendar and reads the traditional chart, which covers nominal ages 18 to 45 — and when your coordinates fall off it, the app says so instead of extrapolating.
If you want to run the parity rule without installing anything, the Mayan gender predictor does it here — same two questions, same coin-flip odds, same disclaimer.