Birth control pills (also called oral contraceptives or "the pill") are used by millions of women in the United States to prevent pregnancy. The pill is safe and effective for most women.
To understand how birth control pills work, you should know what happens during reproduction. A woman has two ovaries, one on each side of the uterus. Each month, one of the ovaries releases an egg into a fallopian tube. This is called ovulation. It typically occurs about 12–14 days before the start of the menstrual period.
A woman can get pregnant if she has sex around the time of ovulation. During sex, the man ejaculates sperm into the vagina. The sperm travel up through the cervix and into the fallopian tube.
When a sperm meets an egg in the fallopian tube, fertilization—union of egg and sperm—can occur. The fertilized egg then moves down the fallopian tube to the uterus. It then attaches to the lining of the uterus and grows into a fetus.
Birth Control Pills
Birth control pills are made of hormones that prevent ovulation. The hormones in the pill cause changes in the cervical mucus and the lining of the uterus. The cervical mucus thickens, which blocks the sperm from entering the cervix. The lining of the uterus thins, making it less likely that a fertilized egg can attach to it. Together, these events make it very unlikely that someone taking the pill will become pregnant.
Each month during ovulation, an egg is released and moves into one of the fallopian tubes. If a woman has sex around this time, an egg may meet a sperm in the fallopian tube and the two will join. This is called fertilization. The fertilized egg then moves through the fallopian tube into the uterus and becomes attached there to grow during pregnancy.
The pill is a very effective form of birth control. When women use the pill correctly, fewer than 1 in 100 will get pregnant over one year. But, about 8 in 100 typical users (8%) will become pregnant. This is because one or more pills may be missed or are not absorbed (due to vomiting, for instance). If this occurs, a backup method of birth control, such as a condom or spermicides, should be used. If pills are missed and a backup method is not used, emergency contraception can be used to prevent pregnancy.