Abstract
Plasma antithrombin III (AT III) was studied longitudinally in 15 subjects (13 patients and 2 volunteers), who used the “morning-after pill” (5 mg ethinylestradiol daily for 5 days). The mean decrease in AT III level in the 13 patients was 17% of the pre-treatment value. From additional observations made in 2 of the patients and in the 2 volunteers it is concluded that this decrease is caused by hemodilution due to salt and water retention rather than by a decreased synthesis or increased consumption.