After the coils wrapped with a fine copper thread had proved themselves, and the purely plastic coils had been superseded, further improvements were explored. Among other things, a hormonal addition was developed, in place of the copper thread.
This idea had already been suggested in the 80s, by the inventor of the Multiload, Willem van Os. However, it was the Schering company that brought it onto the market, in the form of the Mirena. The small hormone depository around the shaft continually releases low quantities of progesterone over a period of five years. On the one hand, this causes the mucus in the cervix to remain viscous and therefore impenetrable for sperm.
On the other hand, the endometrium remains very thin. Therefore, an egg cannot lodge itself, in those very rare cases when fertilization has nevertheless occurred. Additionally, menstruation decreases or does not even take place at all. However, this last consequence has no negative effects; on the contrary, since the body does not have to compensate for the loss of blood.