A. The Role of Female Hormones in Conception

For conception to occur, a sperm must fertilize a mature egg. Specific hormones stimulate the ripening of the egg and its movement from an ovary to the uterus. Hormones are substances formed by one organ that are carried, in the bloodstream, to another organ, where they stimulate that target organ to function. During conception, there is considerable interchange between the ovaries and the pituitary gland located in the brain.

The ovaries produce estrogen, the principal female hormone. It is carried to the pituitary gland, where it stimulates the secretion of follicle-stimulating hormone (FSH). FSH travels to the ovaries where it stimulates the maturation of an unripened egg, or ovum. As the ovum develops, the ovary releases more estrogen.

Increased estrogen in the bloodstream stimulates the pituitary gland to produce large amounts of luteinizing hormone (LH). Increased levels of LH cause a mature egg to be released from an ovary. The process is called ovulation.

After ovulation, the egg passes down the fallopian tube, where it may be fertilized by a sperm. The fertilized egg continues to the uterus, where it attaches to the wall or lining (endometrium). When the ovary releases an egg, its protective coating, the follicle, remains behind, forming the corpus luteum, which produces both estrogen and another hormone, progesterone. The combination of estrogen and progesterone stops production of FSH and LH by the pituitary gland and thickens and strengthens the endometrium, allowing the fertilized egg to develop in the uterus.

If an egg is fertilized, the corpus luteum increases in size, persists for several months, and is critical for sustaining pregnancy. If pregnancy does not occur, the declining levels of progesterone at the end of the normal menstrual cycle cause a complex process in the inner layer of the uterus, the endometrium. As a result, it becomes necrotic and is then shedded, leading to menstruation. With the first day of menstruation, the next cycle in a woman's reproductive life starts.

B. How Do Oral Contraceptives Prevent Pregnancies?

Oral contraceptives employ synthetic hormones that mimic the properties of natural estrogens and/or progesterone to "fool" the female reproductive system. They provide constant levels of an estrogen and/or progestin in the blood, thus suppressing the release of both FSH and LH. Suppression of FSH inhibits maturation of an egg in an ovary. Suppression of LH inhibits release of an egg from the ovary. In addition to the inhibition of ovulation, the constant level of an estrogen and progestin in the body cause insufficient thickening of the endometrium, which prevents attachment of the egg. Progestins also promotes production of thick, opaque mucus, which acts as a barrier to sperm, as sperm can only pass through clear, thin mucus. Progestin is also thought to produce changes in the fallopian tubes that impede movement of the egg toward the uterus. Estrogen and progestin may also alter the pattern of muscle contractions in the tubes and uterus. This effect may interfere with implantation. In case ovulation does occur, which is rare but can happen, these additional effects also help to prevent pregnancies.