Researchers in the Czech Republic have found that women with high levels of the toxoplasma antibody have a 72% chance of giving birth to a boy, compared to the average 51% chance. The Toxoplasma parasite, which can cause birth defects and may trigger miscarriages, is found most frequently in cat feces, and it's why women aren't supposed to be changing the litterbox while they're pregnant.
Pregnant women infected by cat parasite more likely to give birth to boys, say researchers [guardian via boingboing]
Which is one of my arguments against having another kid -- another 9 months of cleaning the catbox!