Combination birth control pills contain both progestin and estrogen to prevent ovulation and create conditions that make it difficult for the sperm to enter into the uterus. The amounts of each hormone vary with the type of combination pill:
- Monophasic pills will deliver the same dose of hormones each day, with no hormones in the inactive pills.
- Multiphasic pills will provide two or three different hormones during the early part of the cycle, before becoming more consistent.
The two hormones in these pills work to prevent pregnancy in multiple ways. The estrogen helps to stop the release of the egg during your monthly cycle, prohibiting ovulation. This prevents pregnancy as there will be no egg for the sperm to fertilize. The progestin in the pill also works to prevent pregnancy but in a different way. It will thicken the mucus around the cervix which can make it difficult for sperm to swim through into the uterus. Both of the hormones also change the lining of the uterus which can make implantation difficult in the unlikely event that you do ovulate and the sperm makes its way to the egg.