There are things in this world which you should avoid if you are planning a pregnancy. A teratogen is a substance or condition which can cause a birth defect. There are relatively few, known teratogens. Not all birth defects are caused by teratogens. In fact, teratogens are probably responsible for less than 10% of human malformations.

Most birth defects caused by teratogens do their damage very early in pregnancy…..around gestational week 2 to 4. This is very early in pregnancy; usually before most women realize they are pregnant. Most teratogens are “species- specific”. This means they cause birth defects in some species of animals but not in others. Many substances (including most pharmeaceutical drugs) are tested for teratogenicity in laboratory animals (like rats, rabbits and mice). It is notable that one of the most potent human teratogens, thalidomide, is not teratogenic in these animals.

Here are the known (and highly suspected) human teratogens:

  • alcohol (ethanol, the kind of alcohol in beer, wine and distilled spirits)
  • aminopterin (a cancer drug)
  • androgens (male hormones)
  • busulfan (Myleran, a drug used to treat leukemia)
  • cyclophosphamide (Cytoxan, a cancer drug)
  • Cytomegalovirus
  • coumarin (an anticoagulant drug)
  • diethylstilbesterol (a hormone drug)
  • Herpes simplex virus, Types I & II
  • hyperthermia (a high body temperature)*
  • lead (?)
  • lithium (used to treat manic-depression)
  • Parvovirus B19 (the cause of Fifth Disease)
  • PCBs & PBBs (polychlorinated biphenyls and polybrominated biphenyls)
  • phenytoin (Dilantin, a drug used to treat epilepsy and seizures)
  • organic mercury
  • Rubella virus (the cause of German measles)
  • tetracycline (an antibiotic drug)
  • thalidomide (a sedative drug)
  • Toxoplasma gondii (the protozoan that causes toxoplasmosis)
  • Treponema pallidum (the bacteria that causes syphilis)
  • trimethadione (and paramethadione, drugs used to treat seizures)
  • valproic acid (a drug used to treat seizures)
  • Varicella virus (the cause of Chicken Pox)
  • Vitamin A in excess (more then 25,000 IU/day) and Vitamin A analogues including:
    • etretinate (Tegison)
    • isotretinoin (Accutane -R, a drug use to treat acne)
  • X-rays (in high doses)

*A high body temperature has been associated with some cranial (head) and nervous system birth defects. The original studies looked at women who used saunas. Other studies have raised suspicion about maternal fevers (temperatures at or above 102 degrees-F). It is believed that the danger period is the early first trimester. The general recommendation is to avoid raising the core body temperature in the first half of pregnancy. Recreational hot tubs are included in the warning. Tepid, bathtub soaks (101 degrees-F or cooler) are probably not a significant cause of elevated core body temperature. A good rule of thumb is that if you begin to feel warm or hot, your core body temperature is rising, and you should get out of the heat source and cool off.