Birth control pills are a very effective contraceptive method when used properly. However, missed doses, starting at the wrong time, or changing the way you take them can significantly reduce their ability to prevent becoming pregnant. It is important that you speak with your doctor
about any concerns you have with regard to how you are taking your birth control pills.
Birth control pills are effective because they prevent ovulation, which is the release of eggs from the ovary into the fallopian tube (where they can be fertilized by sperm). The hormones contained in birth control pills trick the body into thinking that you are already pregnant, so no egg is released by the ovaries, and thus conception cannot happen. To answer your question, if you have already ovulated, then birth control pills cannot prevent this egg release because it has already happened, and they will not be effective in preventing pregnancy.
Although most people ovulate approximately two weeks after the start of their last menstrual period, this can be unpredictable and may vary from month to month. For this reason, if you are taking a combination pill, which contains both progestin and estrogen, you can begin taking this pill up to five days after beginning your menstrual period and you will be protected against pregnancy. If you take it later than that, you must use another form of contraception
for at least seven days until you will be protected (this is because if you have already ovulated, after seven days your egg will be unable to be fertilized and the birth control pill will prevent the next ovulation). If you are taking the progestin only pill, you will begin being protected after 48 hours after beginning the pill. Remember, it is very important that you take the pill at the same time every day. Even a three hour difference in when you take the pill can significantly lower its effectiveness.
Again, if you have specific questions about the timing of your birth control use and its effectiveness, please make an appointment to speak with your doctor.