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" (that's the day the egg is released from the woman's overy — some women can actually feel the event), and sperm mobility and mortality. Sperm can live for several days. 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. The sperm that produces boy babies swims faster, but dies within a day or two.

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 "Copulation Calendar" that shows you when may be the right day to have sex to achieve the gender you desire for your child.