Any drug or chemical substance administered to the mother may cross the placenta unless it is destroyed or altered during this process. Transplacental transport is established at about 8th week of gestation and is dependent on molecular weight and lipid solubility of the substance. Number of drugs prescribed during pregnancy should be restricted to a minimum, with the lowest effective dose given for the shortest time. Drugs should be prescribed in pregnancy only if the benefits outweigh the risks. Teratogenic drugs are drugs which have potential to develop congenital anomalies in the fetus when administered to a pregnant woman. For establishing the teratogenicity of a drug, the defect must be completely characterized, the agent must cross placenta, exposure must occur during the critical period, cause and effect must be biologically plausible and epidemiological studies must be consistent. The time period between day 35 to day 55 from last menstrual period is most crucial because prior to day 35 all cells are totipotent, so any insult before day 35 will result in either a missed abortion or has no effect on the growing embryo. Sometimes drug intake by sexually active men is excreted in their semen and may cause fetal problems. Vaccination of a pregnant woman with inactivated vaccines has not been shown to cause an increase risk to the fetus. Live vaccines are usually contraindicated in pregnancy because pregnancy is an immunocompromised state and there is a potential risk of causing the disease in the mother and the fetus.