The determination of the sex of the future baby will be established from fertilization. It is the sex chromosomes X or Y that determine the sex of the future baby. Since all eggs carry the X chromosome, it is the spermatozoon - X chromosome carrier or Y chromosome carrier - that determines the sex of the baby.
- The mother (ovum) brings an X chromosome, and the father (spermatozoon) his X chromosome: it will be a girl (XX).
- The mother brings an X chromosome, and the father his Y chromosome: it will be a boy (XY).
So there is statistically as many chances to have a girl as a boy.
The special case of twins
It is also during fertilization that twins (or more) will eventually be formed. This peculiarity can have two origins:
During ovulation, two eggs were emitted, and they are fertilized by two different spermatozoa. Two very distinct eggs are formed (hence the term dizygote). We are talking about "false twins", who can be of the same sex or not, and who will look like classic brothers and sisters.
The egg has been fertilized by a spermatozoon, but the egg cell thus created splits in two, forming two different embryos, which will have the same genetic inheritance.
The children will be of the same sex and will be very similar.