Its calculations are based on theories of copulation timing that date back to the days of Empedocles (c 490–430 B.C.). In the western world this hypothesis is over 2,000 years old.

The theory behind copulation timing in selecting the sex of a baby is based on the day of ovulation, and sperm mobility and mortality. Sperm is divided into two cromosomal categories X and Y. The X chromosome sperm which produces girl babies swims slower, but lives longer than the Y chromosome sperm that produces boy babies.

So the timing of ovulation and copulation is a factor in determining which sex your baby will be. For a small fee, GenderSelector.com will provide you with a "ACT" Calendar that shows you when may be the right day to have sex to achieve the gender you desire for your child.