What is the combined monthly hormonal injection? It is monthly injectable form of birth control. It releases two hormones similar to the hormone oestrogen and progesterone that are produced by the ovaries. This form of contraceptive is very reliable and effective. When used correctly and on time, less than 1 pregnancy per 100 women may occur over the first year (5 per 10, 000 women). As commonly used, 3 pregnancies per 100 women occur.