Erythema nodosum associated with pregnancy and oral contraceptives.

Erythema nodosum recurred in a woman during each of her four pregnancies and every time she was started on oral contraceptives. The lesions always disappeared in the fifth month of gestation or when contraceptives were withdrawn. Erythema nodosum is mediated by immune mechanisms, and both pregnancy and oral contraceptive use can interfere with the immune system. The concentrations of oestrogen and progesterone or the ratio between them may be critical to the development of erythema nodosum. The observation that the lesions spontaneously resolved in the fifth month of pregnancy supports this hypothesis.