Dr Smits and his team say their work supports the theory that conception depends on
how viscous, or "sticky", the mucus in a woman's cervix is.
The stickier it is, the harder it is for any sperm to get through.
But Y bearing sperm are lighter, and swim faster.
A woman's age and the pH of the vagina also have an effect on which sperm get through, he said.
Research also shows that male sperm die quicker than female sperm, so researchers say you should have sex closer to ovulation if you want a boy because the male sperm is likely to swim faster to get there and less likely to die from waiting for an egg.
http://news.bbc.co.uk/2/hi/health/4529152.stm