Effects of progestogen oral contraception with norethisterone on blood clotting and platelets.

The effects on clotting tests and platelet function of six months' continuous administration of the 19-norsteroid, progestogen-only contraceptive, norethisterone, have been studied in four groups of women. In a group of women who have not previously taken oral contraceptive no acceleration of clotting or platelet factors was found, but in contrast a tendency to reduced coagulability was observed. Women who had previously been taking combined oestrogen-progestogen preparations showed reduced clotting and platelet parameters when norethisterone was substituted. No changes in clotting or platelets were found in women who changed from 17-acetoxysteroid progestogen chloramadinone acetate or in a group of women started postpartum.