from Greek terato
" or "portent
", + genes
adj. - Of, relating to, or causing malformations of an embryo or fetus, e.g. Thalidomide causing "flipper babies."
Teratogens include several broad categories: pollutants in air or water, medications or recreational drugs, and diseases. For example, just about everyone knows that alcohol use during pregnancy can cause Fetal Alcohol Syndrome and using cocaine while pregnant is also a bad thing, but some acne medications based on vitamin A are also teratogens. If you catch toxoplasmosis or chicken pox--or even have a fever for an extended length of time--during your pregnancy, your child's health may be endangered as well. Phenylketunurics should avoid forms of phenylalanine (including NutraSweet) which can hamper fetal brain development. There are probably electromagnetic teratogens as well; it's a good bet that X-Rays are as bad for embryos as they are for full-grown people.
In general, if you avoid the things that do damage to your body, the body of your baby should also come out well--the few exceptions to this rule are well-known and your doctor can tell you more of them than I can.