The progesterone-only birth control pill. It does not contain estrogen. It works by thickening the cervical mucus which keeps sperm from reaching the egg. It can also help suppress ovulation. Unlike the combination birth control pill, a woman should take the minipill at the same time every day. While it can help decrease menstrual flow and cramping and decrease the risk of uterine cancer, it can also cause irregular period flow, mood changes, and it has decreased effectiveness 22 hours after being taken. The minipill has a failure rate of 9% to 13%, which means 9 to 13 out of 100 women who use it will get pregnant in a year’s time.