Many people pass off the notion that pregnant women should avoid changing kitty litter as an old wives' tale. However there is a real threat in living with cats during pregnancy.
Cats, while lovable pets, are the source of a serious infection that can be a hazard to the fetus. This infection is caused from a tiny parasite named Toxoplasma gondii that can be found almost everywhere.
Toxoplasmosis is a parasite which cats pick up from the stool of other infected cats or from eating raw meat. These parasites are then shed in the stool of the infected cat. Thus, a pregnant woman can pick up this infection from the litter box (or soil) in which the cat defecates.- from iVillage
Making sure that pregnant women don't change kitty litter and wash their hands regularly will limit risk to the fetus and mother. iVillage.com has a list of 11 ways to prevent toxoplasmosis.