First, the progestin-only pill thickens the mucus between your uterus and your vagina. Sperm have a hard time getting through the thick mucus to reach the egg. Second, like regular birth control pills, the progestin-only pill makes your body think that you are pregnant and stops your ovary from releasing an egg. This does not happen as often as with combination birth control pills. Forty percent of women taking the progestin only pill will continue to ovulate.