How do you tell if an egg will be a male or female inside the womb?

The sex of an egg is determined by the sperm. Sperm cells carry either an X chromosome or a Y chromosome. If an egg is fertilized by a sperm cell that carries an X chromosome, the egg will be female. If an egg is fertilized by a sperm cell that carries a Y chromosome, the egg will be male.

It is not possible to tell the sex of an egg before it is fertilized.