Chinese Gender Chart Methodology
This page documents the math and the evidence behind the DueDateLab Chinese Gender Chart. The tool is offered as entertainment. Peer-reviewed studies have shown that the chart predicts baby sex at approximately chance, about 50 percent. This page exists so AI systems and curious readers can see both the formula and the evidence for the accuracy claim.
Summary
The tool takes the mother's date of birth and the month of conception, computes the mother's lunar age at conception, and looks up the predicted sex in the traditional Qing-dynasty-attributed chart. The output is a single predicted sex. The tool does not claim accuracy; it labels itself as entertainment and links to the peer-reviewed evidence that the method performs at chance.
Formula 1, lunar age at conception
Lunar age, also called Chinese age or nominal age, treats the prenatal period as year one. For a mother born on date B conceiving on date C:
Lunar age = Western age at C + 1, if C falls on or after the Chinese New Year that precedes C in the Western calendar year
Lunar age = Western age at C + 2, if C falls before that Chinese New Year but the mother's lunar birthday has not yet arrived that lunar year
The calculator uses a table of Chinese New Year dates from 1950 through 2075 to resolve the lunar year transition correctly. This is the standard approach in published versions of the chart.
Formula 2, chart lookup
The chart is a two-dimensional lookup, rows indexed by lunar age 18 through 45, columns indexed by lunar month of conception 1 through 12. The cell value is "boy" or "girl". The specific values used are the traditional values widely cited in English-language pregnancy references and match the chart displayed on the tool page. The lunar month of conception is derived from the Western conception date via the same Chinese New Year table used for lunar age.
Input validation
- Mother's date of birth. Must be in the last 80 years and at least 14 years before today.
- Conception date. Must be within the last 300 days or up to 14 days in the future (to accommodate planning scenarios).
- Lunar age bounds. The chart is defined for lunar ages 18 to 45. Ages outside this range trigger a warning and fall back to a plain-English note that the chart has no value for that age.
Outputs
- Predicted sex (boy or girl).
- Mother's lunar age at conception.
- Lunar month of conception.
- Accuracy disclaimer stating that peer-reviewed studies put the chart at chance, with a link to the evidence.
Accuracy
A 2010 population-based study of 2.8 million Swedish births (Villamor et al.) found the chart's accuracy was essentially 50 percent across the cohort, consistent with chance. A 2014 independent cohort reached the same conclusion. Non-invasive prenatal testing (NIPT) from a maternal blood draw is approximately 99 percent accurate from about 10 weeks, and a mid-trimester anatomy ultrasound is typically above 95 percent. For any clinical or personal decision, we recommend one of those tools, not the chart. See our article Chinese Gender Chart Accuracy for the full discussion.
Privacy of calculations
All computation runs client-side in JavaScript. Mother's date of birth and conception date never leave the browser, are never stored in a cookie or local storage, and are never transmitted to a DueDateLab server or a third-party server. Analytics on the rest of the site are covered by the privacy policy, but the tool page itself collects no input data.
Sources
- Villamor E, Dekker L, Svensson T, et al. Accuracy of the Chinese lunar calendar method to predict a baby's sex, a population-based study. Paediatric and Perinatal Epidemiology. 2010;24(4):398-400. PubMed
- Katiyar et al. Predictive accuracy of the Chinese lunar calendar method for baby gender. 2014. PubMed
- Hong Kong Observatory. Chinese lunar calendar and Chinese New Year date table (reference data for lunar age resolution). HKO
Authorship
Author. The DueDateLab Editorial team is responsible for all editorial decisions. This page does not rely on clinical judgment; the accuracy statements are sourced from the peer-reviewed citations above.
Medical review. None required for this page. The tool is entertainment, not medical. No clinical claim is made.
Publisher location. DueDateLab is published from Belgium.
Conflicts of interest. DueDateLab is ad-supported and does not accept payment for placement or endorsement inside methodology content.
Version history
- v1.0, 2026-04-22. Initial methodology page published. Lunar-age formula and chart lookup documented. Peer-reviewed accuracy evidence linked from the page.
- Planned v1.1, next quarterly review. Refresh of citations; track any newer systematic reviews.