Teratogenic risk of drugs used in early pregnancy.

Treatment of common illnesses in early pregnancy is complicated because of the risk of teratogenic effects of drugs on the fetus. The period of greatest risk is between the first and eighth week of pregnancy. Since much of this period occurs before a diagnosis of pregnancy is made, care must be used in treatment of common illnesses in all women susceptible to becoming pregnant. Few, if any, drugs have been tested for teratogenicity in controlled clinical trials. Risk must therefore be based on epidemiological studies, individual case reporting and extrapolation from animal studies. Sufficient information is available on commonly used drugs to establish such risks. It is important that drugs of least known risk but adequate efficacy be used in treating intercurrent illness in the first trimester.