What are the chances of a boy or a girl?

Equal numbers of sperm with either an X or a Y chromosome are formed in the testes. After merging with the egg cell-which always has an X chromosome-there should, in theory, be an equal probability of producing male  (XY) or female (XX) offspring. In fact, more boys than girls are born. No one knows exactly why 51.2% of babies are male.
            In exceptional cases,parents may succeed in determining the gender of their child. This can be done because the two types of sperm have different characteristics. Male Y sperm are more mobile, while female X sperm survive longer. When intercourse takes place on the day of ovulation, the faster-moving Y sperm have an advantage, and the chances of concaving a boy are greater.When intercourse takes place few days before ovulation, girl is the more likely result.