A woman becomes pregnant when an egg released from her ovary is fertilized by a man's sperm. The fertilized egg attaches to the inside of a woman's uterus, where it receives nourishment and develops into a baby. Hormones in the woman's body control the release of the egg from the ovary -- called ovulation -- and prepare the body to accept the fertilized egg.

Hormonal contraceptives (the pill, the patch, and the vaginal ring) all contain a small amount of man-made estrogen and progestin hormones. These hormones work to inhibit the body's natural cyclical hormones to prevent pregnancy. Pregnancy is prevented by a combination of factors. The hormonal contraceptive usually stops the body from ovulating. Hormonal contraceptives also change the cervical mucus to make it difficult for the sperm to go through the cervix and find an egg. Hormonal contraceptives can also prevent pregnancy by changing the lining of the womb so it's unlikely the fertilized egg will be implanted.