When is the sex of baby decided?

The sex of a baby is determined at conception when the sperm fertilizes the egg. Each sperm carries either an X or Y chromosome, while the egg carries an X chromosome. If the sperm carries an X chromosome, the resulting embryo will be female (XX), while a Y chromosome from the sperm will lead to a male embryo (XY). This genetic determination occurs before the embryo implants in the uterus.