A baby is conceived when the sperm (produced by the father) fertilises the egg (produced by the mother). The egg carries X chromosome while the sperm can carry X or Y chromosome. If X-carrying sperm fertilises the egg, a baby girl will be conceived and if a Y-carrying sperm fertilises the egg, a baby boy will be conceived.
There is a popular but unfounded theory that you can try to influence the sex of your baby by creating situations that favour the X-carrying sperm over the Y-carrying sperm or vice versa. It has been found that X and Y sperm behave differently. Y (male) sperm swim faster, but are smaller and do not live as long as the larger, slower but more long-lasting X (female) sperm.
To produce a girl, make love frequently up to a few days before ovulation . For a boy make love on or just after ovulation but not too often!
