Your egg contains an X chromosome and you OH's sperm each have either an X or a Y. If an X sperm makes it into the egg you are having a girl, if it's XY you are having a boy. So basically it's all his fault.
There are theories that some women's womb linings and secretions are more friendly to certain types of sperm (x or y) therefore making the women more likely to concieve one gender. Women's bodies can also be very hostile to certain tyes of sperm. Also some scienists sugest that some men's testies create stronger swimmers of one type of cromosome than an other. I think I read that one type are faster swimmers, but die quicker and the other are slower but have a longer lifespan.
There has also been a lot of hype recently sugesting that a woman's diet when conception ocures can effect how friendly her uterus is to the sperm. I think it said more calories in diet = boy. And the article was trying to suggest that UK women's obsession with dietting was causing a slightly higher rate of birth of Girls than boys - but I think there was only 5% in it.
Old wives tales say the more alkaline in your diet the more likely you are to get a boy i think I got this the right way around) and the more acidic - a girl.
Blah blah blah, that's the science. However I think it's pretty much 50/50. You may as well flip a coin to decide which of his swimmers get there first.
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