Antithrombin III and platelets during the normal menstrual cycle and in women receiving oral contraceptives low in oestrogen.

Determinations of platelet number and concentration of antithrombin III (AT-III) were made in samples of plasma collected during one normal or hormone-simulated cycle in 14 young, normal women and in 11 women using an oral contraceptive low in oestrogen. Distinct, individual levels caused the variation of both parameters to be greater within the group than individually. Despite a few exceptions, a slight but statistically significant increase in the platelet number and AT-III level occurred around midcycle in the normal group. In the hormone group the platelet number increased slightly in the treatment period, while the AT-III level showed a corresponding slight decrease. The large decreases in AT-III levels reported by other authors testing oral contraceptives high in oestrogen were not seen in our study.