1. The basic principle of pregnancy to give birth to a boy or a girl. The basic principle of pregnancy to give birth to a boy or a girl is determined by the sex chromosomes carried by the sperm. Male sperm carries XY sex chromosomes, and female sperm carries XX sex chromosomes. When the Y chromosome in a fertilized egg combines with the 2. Factors affecting the baby’s gender 1. Parental age: Research shows that the older the father, the higher the chance of giving birth to a boy;