Section 22. Women's Health Issues

Chapter 241. Biology of the Female Reproductive System

Topics: Introduction | External Genital Organs | Internal Genital Organs | Puberty | Menstrual Cycle | Effects of Aging

Menstrual Cycle

Menstruation is the shedding of the lining of the uterus (endometrium) accompanied by bleeding. It occurs in approximately monthly cycles except during pregnancy and after menopause. Menstruation marks the reproductive years of a woman's life, from the start of menstruation (menarche) during puberty until its cessation (menopause (see Section 22, Chapter 243)).

By definition, the first day of bleeding is counted as the beginning of each menstrual cycle (day 1). The cycle ends just before the next menstrual period. Menstrual cycles range from about 21 to 40 days. Only 10 to 15% of women have cycles that are exactly 28 days. The intervals between periods are usually longest in the years immediately after menarche and before menopause.

The menstrual cycle is regulated by hormones: luteinizing hormone and follicle-stimulating hormone, which are produced by the pituitary gland, and estrogen and progesterone, which are produced by the ovaries. The cycle has three phases: follicular, ovulatory, and luteal.

See the figure Changes During the Menstrual Cycle.

The follicular phase lasts from the first day of bleeding to immediately before a surge in the level of luteinizing hormone. The surge results in release of the egg (ovulation). During this phase, the follicles in the ovaries develop. The follicular phase varies in length, averaging about 13 days of the cycle. This phase tends to become shorter at the end of the reproductive years, near menopause.

At the beginning of the follicular phase, the lining of the uterus (endometrium) thickens with fluids and nutrients designed to nourish an embryo. If no egg has been fertilized, estrogen and progesterone levels decrease, the endometrium is shed, and menstrual bleeding occurs. Menstrual bleeding lasts 3 to 7 days, averaging 5 days. Blood loss during a cycle ranges from ½ to 10 ounces, averaging 4½ ounces. A sanitary pad or tampon, depending on the type, can hold up to an ounce of blood. Menstrual blood, unlike blood resulting from an injury, usually does not clot unless the bleeding is very heavy.

During the first part of the follicular phase, the pituitary gland increases slightly its production of follicle-stimulating hormone. This hormone then stimulates the growth of 3 to 30 follicles, each containing an egg. Later in the phase, as the level of this hormone decreases, only one of these follicles--the dominant follicle--continues to grow. It soon begins to produce estrogen, and the other stimulated follicles begin to degenerate.

The ovulatory phase starts with a surge in the levels of luteinizing hormone and, to a lesser degree, follicle-stimulating hormone. Luteinizing hormone stimulates the dominant follicle to bulge from the surface of the ovary and finally rupture, releasing the egg. (The function of the increase in follicle-stimulating hormone is not understood.)

The ovulatory phase ends with the release of the egg, usually 36 hours after the surge in luteinizing hormone begins. About 12 to 24 hours after the egg is released, this surge can be detected by measuring the luteinizing hormone level in the urine. The egg can be fertilized for only a short period (up to about 12 hours) after its release. Fertilization is more likely when sperm are present in the reproductive tract before the egg is released.

Around the time of ovulation, some women feel a dull pain on one side of the lower abdomen. This pain is known as mittelschmerz (literally, middle pain). The pain may last for a few minutes to a few hours. The pain is felt on the same side as the ovary that released the egg, but the precise cause of the pain is unknown. The pain may precede or follow the rupture of the follicle and may not occur in all cycles. Egg release does not alternate between the two ovaries and appears to be random. If one ovary is removed, the remaining ovary releases an egg every month.

The luteal phase follows ovulation. It lasts about 14 days, unless fertilization occurs, and ends just before a menstrual period. In the luteal phase, the ruptured follicle closes after releasing the egg and forms a structure called a corpus luteum, which produces increasing quantities of progesterone. The function of the corpus luteum is to prepare the uterus in case fertilization occurs. The progesterone produced by the corpus luteum causes the endometrium to thicken, filling with fluids and nutrients in preparation for a potential fetus. Progesterone causes the mucus in the cervix to thicken, making the entry of sperm or bacteria into the uterus less likely. Progesterone also causes body temperature to increase slightly during the luteal phase and remain elevated until a menstrual period begins. This increase in temperature can be used to estimate whether ovulation has occurred (see Section 22, Chapter 254). In the second part of the luteal phase, the estrogen level increases, also stimulating the endometrium to thicken.

In response to the increase in estrogen and progesterone levels, the milk ducts in the breasts increase. As a result, the breasts may swell and become tender.

If the egg is not fertilized, the corpus luteum degenerates after 14 days, and a new menstrual cycle begins. If the egg is fertilized, the cells around the developing embryo begin to produce a hormone called human chorionic gonadotropin. This hormone maintains the corpus luteum, which continues to produce progesterone, until the growing fetus can produce its own hormones. Pregnancy tests are based on detecting an increase in the human chorionic gonadotropin level.