Any substance that can interfere with the normal development of a fetus. Teratogens can result in a spectrum of findings ranging from malformations all the way to intrauterine fetal demise. In blood banking, we primarily worry about teratogens when our blood donors are taking medications with proven teratogenic potential (examples include certain anti-psoriasis, acne, and prostate-shrinking medications). We ask our donors about these medications by name, and if the donor is taking one (or more) of them, we ask them not to donate blood for a set period of time (depending on the medication).