Birth control pill: Effective in first month?

Dear Alice,

What are the statistics for birth control pill failure in the first month that you start taking it?

Dear Reader,

It is difficult to provide exact statistics on failure rates for birth control pills in the first month. However, it is always recommended to use a back-up method of birth control until you’ve completed your first pack, such as a condom, diaphragm, or cervical cap. Back-up BC is necessary because there is a chance that you ovulated (released an egg) before you took your first pill. Therefore, there could be an egg waiting to be fertilized without you even knowing it. After that first pill pack, the hormones present in the birth control pills should prevent you from ovulating altogether (assuming you take the pills correctly), thus contributing to pregnancy prevention.  

Most birth control pills rely on the action of the hormone, estrogen, which essentially inhibits the release of an egg from the ovary. Combination pills that contain estrogen and progestin (a synthetic version of the hormone, progesterone) might provide a little more protection in the first few weeks. This is because progestin decreases the ability of sperm to penetrate the cervical mucus (in order to swim through the uterus and fallopian tubes to fertilize an egg), and alters the lining of the uterus so that it is more difficult for a fertilized egg to implant in the uterus. You can read How do birth control pills work? to learn more.

In addition, it is also a good idea to keep a back-up method of birth control on hand (such as a condom). This is wise to have, just in case you forget to take your pill and want to have sex, or if you're taking antibiotics that could lower the effectiveness of the pill. Check out Antibiotics and hormonal birth control effectiveness for more information.

Last updated Jul 10, 2015
Originally published Apr 11, 1997

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