X and Y sperm
A man's sperm contains 22 chromosomes as well as either an X- or a Y-chromosome. A woman's egg contains 22 chromosomes plus an X-chromosome. The sperm that fertilises the egg determines the baby's sex. If the sperm carries an X-chromosome the baby will be XX and will become a girl. If the sperm carries a Y-chromosome the baby will be XY, a boy.