Antithrombin III in oral contraceptive users and during normotensive pregnancy

Plasma antithrombin III (AT III) was determined in four groups of subjects, by employing an automated chromogenic technique. In 25 women, discontinuing oral contraceptives led to a 9% elevation of AT III, while in 13 women AT III levels fell by 9% after starting with the pill. In 77 normotensive pregnant patients AT III levels were normal during the third trimester and did not differ from control values 6-8 weeks after delivery. Women taking the pill at that time did not have lower AT III levels than those who did not. Furthermore, AT III levels in 414 oral contraceptive users were the same as in 572 controls, when random samples were taken during pill cycle and menstrual cycle. It is concluded that although synthetic estrogens do cause a decrease in AT III levels, this decrease is probably the result of estrogen-induced hemodilution, which may also occur during the normal menstrual cycle. If low dose pills are thrombogenic, mass screening for AT III deficiency will not identify those at risk, with the exception of the rare cases of hereditary AT III deficiency.