The Chinese birth calendar is a chart predicting the gender of a baby based on the mother's age and the month of conception. The original chart was allegedly found in an ancient royal tomb near Beijing and is currently located at the Institute of Science in Beijing. The Chinese birth calendar can be found on numerous websites, and accuracy is variously predicted at anywhere from 50% to 99%.
The Chinese birth calendar has ages from 18 to 45 on one axis and months on the other axis, and to use it, one locates the box corresponding to the appropriate maternal age and month of conception. Each box indicates either male or female gender. There are some differences between Chinese and Western methods of reckoning time that may skew results.
First, the mother's lunar age should be used. This is calculated by determining the number of lunar years in which one has lived, so it is always a year or two more than one's age by Western reckoning. Lunar age increases at each lunar New Year, which takes place between January 21 and February 20, rather than on the date of birth.
In addition, Chinese months do not correspond exactly with Western months. Rather, each begins on a new moon. Also, the current Chinese calendar is corrected to correspond with the solar year; for example, there is a thirteenth leap month every two or three years. If the Chinese birth calendar is ancient as it is claimed to be, it would be based on an older lunar system, making all the dates slightly off. All Chinese birth calendars available on the Internet use Western months, so it is difficult to tell if one is using it correctly.
Most people view the Chinese birth calendar as a mere novelty. The supposed scientific basis behind it is that a woman's vaginal secretions at different times in her life are more or less hospitable to the slightly smaller Y-chromosome sperms, which are responsible for the conception of a male fetus. However, much more scientific and accurate methods, such as ultrasound, are available today if a woman wishes to know the gender of her baby.