When she is a fetus in her mother’s body, so technically the eggs are created even before she is born and as she grows a girl keeps on losing the eggs, every month through menstruation, but retaining the same fertile quality. In the initial stages, the embryo, irrespective of whether it is a boy or a girl, looks exactly the same, with two sets of tube, each carrying male and female genitals. The one chromosome pair, among the 23 pairs, will be determining the sex of the baby, as an embryo with two X chromosomes become a girl, and a combination of X and Y chromosome will become a boy.