Drugs should be avoided during pregnancy unless essential because of possible adverse effect on the fetus. Drugs may pass through the placenta and cause harm, as the enzyme systems responsible for their degradation are undeveloped in the fetus. While many drugs are harmless, examples of problems include warfarin, which may induce fetal and placental haemorrhage, and thiazides which may produce THROMBOCYTOPENIA in the newborn. Many progestogens have androgenic side-effects and their administration to a mother for the purpose of preventing recurrent abortion may produce VIRILISATION of the female fetus. Tetracycline administered during the last trimester commonly stains yellow the deciduous teeth of the child.

The other dangers of administering drugs in pregnancy are the teratogenic effects, for example as was seen decades ago with thalidomide (see TERATOGENESIS). It is understandable that a drug may interfere with a mechanism essential for growth and result in arrested or distorted development of the fetus and yet cause no disturbance in the adult, in whom these processes of differentiation and organisation have ceased to be relevant. The susceptibility of the embryo will depend on its stage of development when the drug is given. The stage of early differentiation – that is, from the beginning of the third week to the end of the tenth week of pregnancy – is the time of greatest susceptibility. After this time the risk of congenital malformation from drug treatment is less, although death of the fetus can occur at any time.