It contains very small amounts of only one kind of hormone, a progestin. Again, they contain only one-half to one tenth as much progestin as combined oral contraceptives. They do not contain estrogen. They do not reduce milk production.
Progestin works in a manner similar to breast-feeding.
It thickens the cervical mucus, making it difficult for sperm to pass through. Progestin-only oral contraceptives do not disrupt existing pregnancy. Pregnancy rates for women not breast-feeding are not available. There is only about one pregnancy per 100 women in the first year of use. Mistakes in pill taking lead to pregnancy more often than with combined oral contraceptives.