There is preliminary research, published in Nature, that foods with extra Propianic Acic in them may pass that acid through a pregnant woman's body into a fetus. The chemical is used to increase the shelf life of some foods. This chemical may contribute to causing autism. The chemical does occur naturally in humans, but eating foods with it while pregnant might push the amount of the chemical to harmful levels in fetuses. The next step to determine if this actually happens is to test mice.

Link: Nature