It can cause many anatomical defects, such as heart malformations, if the fetus is infected during the first trimester of pregnancy. A rubella infection occurring later is less serious. There is a great deal of individual variation in response to the size of a dose of an agent and to particular agents. Alcohol is a good example. Large amounts appear to have no effect on some fetuses, while other fetuses may be harmed by low amounts.
Animal studies provide much of our information about possible harmful agents.
This information can be helpful but cannot always be applied directly to humans. Other information comes from situations in which women were exposed who did not know they were pregnant or who did not know a particular medication could be harmful. Information gathered from these instances is difficult to apply directly to a particular pregnancy. A list of known teratogens and the effects they may have on an embryo or fetus follows. If you have taken any of these substances, discuss them as soon as possible with your doctor for your peace of mind.