Is the Chinese Gender Calendar Accurate?

What the studies found

The most widely cited test was published in the Canadian Medical Association Journal in 2010. Researchers applied the chart retrospectively to a large set of births with known conception dates and known outcomes. It predicted correctly about half the time, statistically indistinguishable from flipping a coin.

Smaller analyses have reached the same place. Nobody has produced a replicated result showing the chart performing meaningfully above chance, and there is no proposed mechanism by which a mother’s age and the phase of the moon could influence which of two sperm types reaches the egg first.

Why 50 percent feels higher than it is

A binary prediction that is right half the time generates a great deal of convincing anecdote. Three things amplify it:

• Survivorship in the retelling. A friend whose result was correct tells you about it. A friend whose result was wrong usually does not bring it up.
• Confirmation after the fact. Most people check the chart once they already suspect an answer, then remember the agreement rather than the disagreement.
• Retry until it fits. Different sources disagree about how to compute lunar age. Try two or three and one will usually match reality, which is then the one people quote.

The chart is close to an even split by design

Across all 336 cells of the traditional chart, 164 predict a boy and 172 predict a girl, or 48.8 percent boys. Real-world births run at roughly 51 percent boys. A near-even grid applied to a near-even reality produces roughly even accuracy, no matter how the cells are arranged.

The origin story is also unverified

The chart is usually described as a 700-year-old document recovered from a Qing dynasty royal tomb and held in the Beijing Institute of Science. No published catalogue entry or scholarly reference for such an artefact has ever been produced, and the Qing dynasty itself only began in 1644. The provenance is best read as part of the folklore.

What actually works

If you want to know rather than guess, two methods are genuinely reliable:

• NIPT (cell-free DNA blood test), available from around 10 weeks, is over 99 percent accurate for fetal sex.
• Anatomy scan ultrasound, usually at 18 to 20 weeks, is highly reliable when the baby is positioned well.

Both are conversations to have with your midwife or doctor. Availability and local rules around sex disclosure vary by country.

So why use the chart at all?

Because it is fun, and because being fun is enough. It costs nothing, it makes a nice page in a baby book, and it gives a gender-reveal party something to argue about. The problem is never people enjoying a tradition; it is sites presenting it as a test. This one will not.