Estrogen prevents the egg from maturing and being released from the ovary. No egg - no pregnancy. Progesterone causes changes in the lining of the uterus and in the consistency of the cervical mucus. The Mini-Pill contains progesterone only.
Periods are more regular.
Can be used for women over age 40. Does not interfere with lovemaking. Does not protect against STD's. Must be taken daily. May need to use a barrier method if pill missed or antibiotics are taken.