Level 1: The scientific basis for pregnancy to give birth to a boy or a girl What is the scientific basis for pregnancy to give birth to a boy or a girl? Human sex is determined by the sex chromosomes carried by the father\’s sperm. If the sperm carries the X chromosome, the fetus is a girl; if the sperm carries the Y chromosome, the fetus is a boy. Therefore, if you want to know that you are going to be pregnant with a boy,