Mayan gender predictor: the rule, the working and the evidence

How the Mayan gender predictor works

The rule needs two numbers. The first is the mother's age at conception, in completed years. The second is the number of the month in which conception took place, counting January as 1 and December as 12. Many sites call it the Mayan calendar gender calculator, although it has nothing to do with the Maya calendars.

Put another way, the rule gives a girl when age plus month is an even number. At any age, six months of the year give a girl and six give a boy, so over a year the rule divides evenly by construction. A change of one day can move conception into the next month, or past a birthday, and reverse the answer.

Gender prediction based on a due date or last period

A gender predictor by due date works back to conception first. Conception is estimated as 266 days before the due date or, for a gender calculator by last menstrual period, as the first day of the last period plus the cycle length minus 14 days. Both estimates are uncertain by several days, so the calculator applies the rule to every day conception could have fallen on and shows each answer that window allows. The date arithmetic adds nothing to the rule; it only supplies the date the rule needs.

Where the so-called Mayan rule comes from

No historical, archaeological or ethnographic source records it. The rule depends on the twelve-month Gregorian calendar and on counting age in years, and the Maya used neither. Their calendars counted a 260-day ritual cycle, the Tzolk'in, and a 365-day year, the Haab', of eighteen 20-day months followed by five further days. No Maya month corresponds to April, or to the number 4. The rule is a modern invention given a Maya name. The Maya are a living people, and some highland communities still keep the Tzolk'in; the rule should not be presented as their belief.

Versions of the rule disagree

Of 17 publishers we compared in September 2026, 15 give the rule as it is stated here, one reverses it and one does not say. Eight compare the mother's age with the month of conception and six with the year of conception; two contradict themselves. Under the year version, every mother of the same age parity receives the same answer in a given year, and every answer reverses the following year. This calculator uses the month version, the most widely published, and does not offer the others: a choice of versions cannot make a rule with no mechanism more accurate.

How accurate is the Mayan gender predictor?

People often ask which gender predictor is the most accurate. Among the folk methods, none is. No study has tested the Mayan rule directly. The nearest evidence comes from a study of the Chinese lunar calendar method, a similar folk chart, checked against 2,840,755 births in Sweden between 1973 and 2006: its predictions agreed with the babies' sex no more often than chance (Villamor and colleagues, 2010).

The Mayan rule has no plausible mechanism. A baby's sex is set at fertilization, by whether the sperm carries an X or a Y chromosome, and neither the mother's age nor the calendar month has any bearing on it. About 105 boys are born for every 100 girls (United Nations, World Population Prospects 2024), so guessing "boy" every time would be right slightly more often than this rule.

Reliable ways to find out the baby's sex

• A blood test from 10 weeks. Cell-free DNA screening, often called NIPT, can report the baby's sex from about 10 weeks of pregnancy. It is highly accurate but not certain. In the UK the NHS offers it only after a higher-chance screening result, and the NHS test does not report sex; private tests do. Tests offered before 7 weeks, and urine tests, are not reliable.
• The mid-pregnancy scan. In the US, the anatomy scan at 18 to 22 weeks can usually show the sex. In the UK, sex is not part of the 20-week screening scan (18 to 21 weeks), and whether the sonographer will say depends on the hospital's policy.
• Not by invasive testing. Chorionic villus sampling and amniocentesis are diagnostic tests offered for medical reasons, and each carries a small risk of miscarriage, about 1 in 200 or lower (NHS). They are not done to learn the sex.

Other ways people guess the sex

• The baby's heart rate. The folk rule says that more than 140 beats a minute means a girl. Studies of 477 and 655 pregnancies found no meaningful difference between boys and girls in early pregnancy (McKenna and colleagues, 2006; Bracero and colleagues, 2016), and both sexes average more than 140 in the first trimester.
• The nub theory. Trained sonographers measuring the angle of the genital tubercle on a correctly aligned scan can assign sex accurately at 12 to 13 weeks, and less reliably at 11 weeks. That is a clinical scan finding. Judging a screenshot at home is a different practice, and it has not been studied.
• The Ramzi method. The claim that the side of the placenta at 6 weeks shows the sex comes from a 2011 online paper that was not peer reviewed and has not been confirmed. Left and right on a scan image also depend on how the probe is held.
• Timing intercourse. A study of 221 women trying to conceive found that the timing of intercourse relative to ovulation made no difference to the baby's sex (Wilcox and colleagues, 1995).
• The ring test and the baking soda test. No study supports either. Urine changes with diet, fluid intake and infection, not with the baby's sex.