Combination
Estrogen and progesterone combined prevent ovulation, thicken your cervical mucus and thin the lining of the uterus. This type of pill is called the combination pill. The levels of estrogen in the combination pill are much lower than when the pill debuted, and ovulation may still occur in 2 to 10 percent of women, according to Epigee.org, a site that focuses on women's health. However, you can minimize this potential by making sure that you take the pill correctly -- at the same time every day -- to minimize any potential fluctuation in your hormones.
By suppressing ovulation, this pill prevents an egg from being released and potentially fertilized by any sperm. However, as noted, ovulation may still occur. Thickened cervical mucus, however, makes it difficult for sperm to pass into the cervix, making fertilization unlikely. And given that this pill also thins the lining of the uterus, it prevents a fertilized embryo from implanting in the wall of the uterus.
This pill prevents pregnancy in 99 out of 100 women when taken correctly, notes Planned Parenthood. And for those who don't take it correctly, Planned Parenthood notes that is still effective for 92 out of 100 women.
Mini-Pill
The progestin-only pill (a synthetic form of progesterone) is often given to nursing mothers who wish to prevent pregnancy and are breastfeeding. This pill minimizes the risk of passing excess estrogen to your nursing baby, yet still prevents pregnancy. It works primarily by thickening your cervical mucus and must be taken every day at the same time, as it is less effective than the combination pill since it does not contain estrogen. It may also prevent ovulation, though ovulation is not suppressed for all women. Some women experience no period while on this form of birth control pill, others experience irregular bleeding. In addition, for some women, it also thins the lining of the uterus.
This pill is equally effective as the combination pill if it is taken at exactly the same time every day. Contact your doctor if you begin having breakthrough bleeding (bleeding at times other than your normal menstrual cycle) while taking this pill, especially if your menstrual period has ceased while on the mini-pill. She may switch you to a combination pill or recommend a secondary form of birth control.
