How does the fetus change gender in third week?

The fetus does not change gender in the third week. The sex of a fetus is determined at the moment of conception, when the sperm fertilizes the egg. The sperm cell carries either an X or a Y chromosome, while the egg cell always carries an X chromosome. If the sperm cell carries an X chromosome, the baby will be female (XX). If the sperm cell carries a Y chromosome, the baby will be male (XY).