Sometimes precautions fail and unplanned pregnancies occur, even when you’re careful. While the birth control pill is quite effective, you can get pregnant on the pill. With perfect use, the pill is more than 99 percent effective; however, typical use failure rates are around 8 percent. Daily life may interfere, causing you to miss pills, and other medications can lower the pill’s effectiveness. In some cases, even with perfect use, pregnancy results.
While all birth control pills are hormonal contraceptives taken daily, there are some differences. Combination birth control pills rely upon both estrogen and progesterone to prevent pregnancy. Pill doses may vary, and your doctor can help you choose the one that is best for you. Progesterone-only oral contraceptives, called mini-pills, are somewhat less effective but may be used when estrogen cannot be tolerated. The birth control patch and ring rely upon progesterone and estrogen as well, while the birth control shot is progesterone only.
Combination birth control pills and patches, as well as progesterone-only contraceptives, work in three distinct ways. First, the hormones prevent ovulation. The progesterone thickens the cervical fluid, acting as a barrier between the cervix and sperm. Finally, the hormones thin and alter the uterine lining. While blocking ovulation is the primary means of preventing pregnancy, the other effects of hormonal contraceptives reduce the overall risk of pregnancy if you do ovulate.
The pill must be taken daily in order to provide the most effective protection against pregnancy. If you are taking a combination pill and forget your pill once, you can simply take it as soon as you remember, taking two pills if necessary. If you forget more than one pill, you should speak to your health care provider about getting back on track and be sure to use a backup contraceptive. Similar precautions should be taken if you miss pills due to illness or cannot keep your birth control pills down due to diarrhea or vomiting. Missed pills are the most common cause of birth control failure. While the patch, ring, and shot have less risk of user error, do be sure to use a backup contraceptive if you are late with your patch or ring, or miss an appointment for your birth control shot.
Certain medications can lower the effectiveness of birth control, increasing the risk of pregnancy. The antibiotic rifampin makes the pill less effective, as do some oral anti-fungal medications. The herbal supplement St. John’s Wort also reduces the efficacy of oral contraceptives. Some HIV or seizure medications can also conflict with the birth control pill. Be sure to tell your doctor that you are taking the pill so appropriate medications can be chosen or so you can know if you need a backup contraceptive that cycle to reduce the risk of an unplanned pregnancy. These medication interactions can apply to all forms of hormonal contraception.
Women with a body mass index or BMI of more than 27.3 may be at higher risk of pregnancy when on birth control pills or other hormonal contraceptives. The risk of pregnancy while on the pill more than doubles in heavier women. A recent study by Dr. Alison Edelman shows that it takes significantly longer for the hormones in birth control pills to reach an effective clinical dose. Overweight women should discuss the appropriateness of birth control pills with their physicians and consider using a backup birth control method for at least a portion of the menstrual cycle.