There are 50% chances that a girl may be born and 50% chances that a boy may be born.
It can be explained as follows:
Most human chromosomes have a maternal copy and a paternal copy. We have 22 such chromosomes. One pair of chromosomes called sex chromosomes is odd in not always being a perfect pair. Women have a perfect pair of sex chromosomes, both called X. (XX)
But men have a mismatched pair of sex chromosomes in which one is normal sized – X chromosome while the other is a short one called Y chromosome. (XY)
A child receives one chromosome from mother which is essentially X chromosome.
A child who inherits an X chromosome from her father will be a girl, and one who inherits a Y chromosome from him will be a boy.