Expectant mothers know the usual cautions about their condition. Don’t drink, don’t smoke, don’t clean out the cat box, take care to avoid falls or other disturbances.

But one of the big mysteries of conception is what women should do before getting pregnant. After all, the pre-pregnancy days include going to school, working, potentially indulging in risky behavior and generally living life as normal people live it. Can any of that have an effect on fetuses?

The answer is largely a mystery. The March of Dimes notes that unknown factors cause 70 percent of birth defects. While most of the known causes of defects can be traced to genetics, that leaves a wide swath of potential factors that may put future pregnancies at risk. And one of the prime suspects is potential exposure to toxins before becoming pregnant.

Researchers call substances that can cause birth defects "teratogens." The American College of Obstetrics and Gynecology bulletin on teratology claims that the length of exposure to teratogens, as well as the level, may play a role. The stage of pregnancy is also a factor.

There is a long list of drugs and chemicals that can affect pregnancy or alter a mother’s health in pre-pregnancy. Among them are alcohol, testosterone derivatives, androgens, coumarin derivatives, folic acid antagonists, cocaine, lead, lithium, organic mercury, phenytoin, tetracycline, valproic acid, Vitamin A and its derivatives, rubella, syphilis, toxoplasmosis and varicella, among others.