Contraceptive progestogens have a series of effects on the endometrium that depend on the existence of oestrogen priming and therefore on the time of administration, the route through which the hormone is released to the body (systemic or locally in utero) and the available daily dose. The effects of a contraceptive progestogen can be divided into two main categories: changes in the endometrial structure and vascularization and alterations of the menstrual bleeding pattern. Whereas orally

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... stered progestogens usually cause endometrial decidualization and an important stromal reaction, the i.m., or local, intrauterine delivery is more apt to cause atrophia. Finally, all progestogens, when given alone at contraceptive doses (and irrespective of their mechanism of action), cause some disruption of menstrual bleeding patterns. This is maximal with injectable, long-acting progestogens, such as depot-medroxyprogesterone acetate and norethisterone enantate.