How is it possible for Baby-Comp to show on the fertile days whether a baby is more likely to be a boy or a girl?
The spermatozoon that reaches the ovum first and then fertilizes it determines the sex of the child. If this is an X spermatozoon the baby will be a girl, if it is a y spermatozoon it will be a boy. There is almost the same number of X and Y spermatozoa in the man's semen meaning that in the first place the chances are 50:50. New findings have indicated that the two types of spermatozoa do not just differ in their hereditary factors but also in other characteristics. According to this research, the sex of a child depends on whether sexual intercourse took place on the same day as ovulation or some days before.

One possible explanation for this observation is that either the X or Y spermatozoon swims faster than the other, is larger or smaller, heavier or lighter but becomes exhausted more rapidly and therefore loses its ability to fertilize. Considerable energy is required for the sperm to first reach its meeting place with the egg, then to penetrate the thick capsule of the ovum before finally fertilizing the egg cell. After some time has elapsed only the hardier, but somewhat slower spermatozoa, have sufficient energy to achieve fertilization.

This has resulted in the following conclusions being drawn: if sexual intercourse occurs on the same day as ovulation, there is a greater chance that one of the faster spermatozoa will arrive first and fertilize the egg. However, if intercourse takes place more than two days before ovulation, the likelihood is greater that only the hardier type of spermatozoa will still be able to fertilize the egg.