I think it's genetic, and influenced by your general health (in some cases), and the level of stress you're under, and sometimes, it's just totally random.  Old wives tales about guessing the gender based on morning sickness or how you're carrying baby drive me NUTS!!     Our sonographer said she was 99.8% sure ours is a little girl (yeah, I know that's not a real statistic and she just plucked it from thin air, but still, she said she doesn't declare the child's gender unless she's very sure of it, and even I thought she definantly looked like a girl in the scan), and since then, people have felt it necessary to say they think it's a boy, because of how I'm carrying (what - in my belly as opposed to my shoulders or knees?!?!), or because of how long the morning sickness lasted (I had the standard issue MS - from 6wks pregnant to 12wks).

It's just fun trying to guess a baby's gender, but once you've found out (or as good as), it's a bit mean to go about declaring to the mum that in your uninformed opinion, it's all wrong and she'll end up with a boy in pink clothes!  (sorry - that all turned into a bit of a rant, didn't it!  My bad).