A baby is conceived from the fertilisation of a sperm and an egg. The egg only carries a X chromosome while a sperm can carry either a X or Y chromosome from the father. If a X-carrying sperm fertilises the egg, a baby girl (XX) will be conceived and if a Y-carrying sperm fertilises the egg, a baby boy (XY) will be conceived. Therefore, it is evident that the father’s sperm determines the gender of the offspring.
As in all things, various myths do exist and have no scientific basis at all. They include things such as sleeping positions and having intercourse on certain calendar days, etc. These have not been shown to influence a baby’s gender.