Birth control pills are 99 percent effective at preventing pregnancy when used consistently and correctly. If you do not use the birth control pill consistently (every day), or correctly (about the same time of day), your birth control pill's effectiveness is decreased.
How much are my pills' effectiveness decreased?
It is impossible to provide an exact percentage, but the basic formula is: the more pills you miss + (plus) the more frequent intercourse you have = (equals) the greater chances of pregnancy.
Why can't I miss a few pills if I've taken them for years? Is it a big deal if I miss more than one pill?
Yes, if you miss more than one pill it is a big deal, as each cycle counts as its own, and makes your pill-taking history irrelevant, as your body requires a consistent supply of the synthetic hormones to work effectively for each cycle alone. Every individual cycle presents an opportunity for ovulation, and this presents the opportunity for pregnancy. It is helpful to understand the basic mechanism of hormonal contraception in your body, as it works in one (or all) of these ways:
- Prevents ovulation (maturation and release of an egg)
- Alters the lining of the uterus (makes this environment inhabitable for a fertilized egg)
- Alters the cervical mucus (sperm has difficulty "swimming" through your cervix)
The pill essentially stops your body from these natural functions by way of introducing synthetic hormones into your body, resulting in your body's decreased production of its own natural hormones. Without the pill, your natural hormones work throughout your cycle to begin maturing an egg, release the egg (ovulation), and otherwise make a cozy environment for the sperm to enter and a fertilized egg to implant in the uterus. These processes are working continuously from cycle to cycle, and the birth control pills work to deter these processes on a daily basis.