The female reproductive system consists of the external and internal genital organs. The breasts are sometimes considered part of the reproductive system (see Breast Disorders: Introduction). However, other parts of the body also affect the development and functioning of the reproductive system. They include the hypothalamus (an area of the brain), the pituitary gland (located at the base of the brain, directly below the hypothalamus), and the adrenal glands (located on top of the kidneys). The hypothalamus orchestrates the interactions among the genital organs, pituitary gland, and adrenal glands (see Biology of the Endocrine System: Major Endocrine Glands). These parts of the body interact with each other by releasing hormones. Hormones are chemical messengers that control and coordinate activities in the body. The hypothalamus produces gonadotropin-releasing hormone, which stimulates the pituitary gland to produce luteinizing hormone and follicle-stimulating hormone. These hormones stimulate the ovaries to produce the female sex hormones, estrogen and progesterone, and some male sex hormones (androgens). (Male sex hormones stimulate the growth of pubic and underarm hair at puberty and maintain muscle mass in girls as well as boys.) After childbirth, the hypothalamus signals the pituitary gland to produce prolactin, a hormone that stimulates milk production. The adrenal glands produce small amounts of female and male sex hormones.

Around menopause (see Menopause: Introduction), changes in the genital organs occur rapidly. Menstrual cycles stop, and the ovaries stop producing estrogen. After menopause, the tissues of the labia minora, clitoris, vagina, and urethra thin (atrophy). This thinning can result in chronic irritation, dryness, and a discharge from the vagina. Vaginal infections are more likely to develop. Also after menopause, the uterus, fallopian tubes, and ovaries become smaller.

With aging, there is a decrease in the amount of muscle and connective tissue, including that in muscles, ligaments, and other tissues that support the bladder, uterus, vagina, and rectum. As a result, the affected organs may sag or drop down (prolapse), sometimes causing a feeling of pelvic pressure or fullness, difficulty urinating, loss of control of urination or bowel movements (incontinence), or pain during sexual intercourse. Women who have had many children are more likely to have such problems.

Because there is less estrogen to stimulate milk ducts, the breasts decrease in size and may sag. The connective tissue that supports the breasts also decreases, contributing to sagging. Fibrous tissue in the breasts is replaced with fat, making the breasts less firm.

Despite these changes, many women enjoy sexual activity more after menopause, possibly because they are no longer able to become pregnant. In addition, after menopause, the ovaries and adrenal glands continue to produce male sex hormones. Male sex hormones help maintain the sex drive, slow the loss of muscle tissue, and contribute to an overall sense of well-being.

The external genital organs include the mons pubis, labia majora, labia minora, Bartholin's glands, and clitoris. The area containing these organs is called the vulva. The external genital organs have three main functions:

• Enabling sperm to enter the body
• Protecting the internal genital organs from infectious organisms
• Providing sexual pleasure

The mons pubis is a rounded mound of fatty tissue that covers the pubic bone. During puberty, it becomes covered with hair. The mons pubis contains oil-secreting (sebaceous) glands that release substances that are involved in sexual attraction (pheromones). The labia majora (literally, large lips) are relatively large, fleshy folds of tissue that enclose and protect the other external genital organs. They are comparable to the scrotum in males. The labia majora contain sweat and sebaceous glands, which produce lubricating secretions. After puberty, hair appears on the labia majora.

The labia minora (literally, small lips) can be very small or up to 2 inches wide. The labia minora lie just inside the labia majora and surround the openings to the vagina and urethra. A rich supply of blood vessels gives the labia minora a pink color. During sexual stimulation, these blood vessels become engorged with blood, causing the labia minora to swell and become more sensitive to stimulation.

The area between the vaginal opening and the anus, below the labia majora, is called the perineum. It varies in length from almost 1 to more than 2 inches (2 to 5 centimeters).

The labia majora and the perineum are covered with skin similar to that on the rest of the body. The skin is thick, dry, and sometimes scaly. In contrast, the labia minora are lined with a mucous membrane, whose surface is kept moist by fluid secreted by specialized cells.

The opening to the vagina is called the introitus. The vaginal opening is the entryway for the penis during sexual intercourse and the exit for menstrual blood and vaginal discharge as well as a baby. When stimulated, Bartholin's glands (located beside the vaginal opening) secrete a thick fluid that supplies lubrication for intercourse. The opening to the urethra, which carries urine from the bladder to the outside, is located above and in front of the vaginal opening.

The clitoris, located between the labia minora at their upper end, is a small protrusion that corresponds to the penis in the male. The clitoris, like the penis, is very sensitive to sexual stimulation and can become erect. Stimulating the clitoris can result in an orgasm.

The internal genital organs form a pathway (the genital tract). This pathway consists of the following:

• Vagina (part of the birth canal), where sperm are deposited and from which a baby can emerge
• Uterus, where an embryo can develop into a fetus
• Fallopian tubes (oviducts), where a sperm can fertilize an egg
• Ovaries, which produce and release eggs

Sperm can travel up the tract, and eggs down the tract.

At the beginning of the tract, just inside the opening of the vagina, is the hymen, a mucous membrane. In virgins, the hymen usually encircles the opening like a tight ring, but it may completely cover the opening. The hymen helps protect the genital tract but is not necessary for health. It may tear at the first attempt at sexual intercourse, or it may be so soft and pliable that no tearing occurs. The hymen may also be torn during exercise or insertion of a tampon or diaphragm. Tearing usually causes slight bleeding. In women who have had intercourse, the hymen may be unnoticeable or may form small tags of tissue around the vaginal opening.

Vagina: The vagina is a narrow, muscular but elastic organ about 4 to 5 inches long in an adult woman. It connects the external genital organs to the uterus. The vagina is the main female organ of sexual intercourse. The penis is inserted into it. It is the passageway for sperm to the egg and for menstrual bleeding or a baby to the outside.

Usually, there is no space inside the vagina unless it is stretched open—for example, during an examination, sexual intercourse, or childbirth. The lower third of the vagina is surrounded by elastic muscles that control the diameter of its opening. These muscles contract rhythmically and involuntarily during orgasm.

The vagina is lined with a mucous membrane, kept moist by fluids oozing from cells on its surface and by secretions from glands in the cervix (the lower part of the uterus). A small amount of these fluids may pass to the outside as a clear or milky white vaginal discharge, which is normal. During a woman's reproductive years, the lining of the vagina has folds and wrinkles. Before puberty and after menopause (if the woman is not taking estrogen), the lining is smooth.

Uterus and Cervix: The uterus is a thick-walled, muscular, pear-shaped organ located in the middle of the pelvis, behind the bladder, and in front of the rectum. The uterus is anchored in position by several ligaments. The main function of the uterus is to sustain a developing fetus. The uterus consists of the cervix and the main body (corpus).

The cervix is the lower part of the uterus, which protrudes into the upper end of the vagina. It can be seen during a pelvic examination. Like the vagina, the cervix is lined with a mucous membrane, but the mucous membrane of the cervix is smooth.

Sperm can enter and menstrual blood can exit the uterus through a channel in the cervix. The channel is usually narrow, but during labor, the channel widens to let the baby through. The cervix is usually a good barrier against bacteria, except around the time an egg is released by the ovaries (ovulation), during the menstrual period, or during labor. Bacteria that cause sexually transmitted diseases (see Sexually Transmitted Diseases: Introduction) can enter the uterus through the cervix during sexual intercourse.

The channel through the cervix is lined with glands that secrete mucus. This mucus is thick and impenetrable to sperm until just before ovulation. At ovulation, the consistency of the mucus changes so that sperm can swim through it and fertilization can occur. At this time, the mucus-secreting glands of the cervix can store live sperm for up to about 5 days, but occasionally slightly longer. These sperm can later move up through the corpus and into the fallopian tubes to fertilize an egg. Almost all pregnancies result from intercourse that occurs during the 3 days before ovulation. However, pregnancies sometimes result from intercourse that occurs up to 6 days before ovulation or during the 3 days after ovulation. For some women, the time between a menstrual period and ovulation varies from month to month. Consequently, pregnancy can occur at different times during a menstrual cycle.

The corpus of the uterus, which is highly muscular, can stretch to accommodate a growing fetus. Its muscular walls contract during labor to push the baby out through the cervix and the vagina. During the reproductive years, the corpus is twice as long as the cervix. After menopause, the reverse is true.

As part of a woman's reproductive cycle (which usually lasts about a month), the lining of the corpus (endometrium) thickens. If the woman does not become pregnant during that cycle, most of the endometrium is shed and bleeding occurs, resulting in the menstrual period.

Fallopian Tubes: The two fallopian tubes, which are about 2 to 3 inches (about 5 to 7 centimeters) long, extend from the upper edges of the uterus toward the ovaries. The tubes do not directly connect with the ovaries. Instead, the end of each tube flares into a funnel shape with fingerlike extensions (fimbriae). When an egg is released from an ovary, the fimbriae guide the egg into the relatively large opening of a fallopian tube.

The fallopian tubes are lined with tiny hairlike projections (cilia). The cilia and the muscles in the tube's wall propel an egg downward through the tube to the uterus. The egg may be fertilized by a sperm in the fallopian tube (see Normal Pregnancy: Fertilization).

Ovaries: The ovaries are usually pearl-colored, oblong, and about the size of a walnut. They are attached to the uterus by ligaments. In addition to producing female sex hormones (estrogen and progesterone) and male sex hormones, the ovaries produce and release eggs. The developing egg cells (oocytes) are contained in fluid-filled cavities (follicles) in the wall of the ovaries. Each follicle contains one oocyte.

Menstruation is the shedding of the lining of the uterus (endometrium) accompanied by bleeding. It occurs in approximately monthly cycles throughout a woman's reproductive life, except during pregnancy. Menstruation starts during puberty (at menarche) and stops permanently at menopause (see Menopause: Introduction).

By definition, the menstrual cycle begins with the first day of bleeding, which is counted as day 1. The cycle ends just before the next menstrual period. Menstrual cycles normally range from about 25 to 36 days. Only 10 to 15% of women have cycles that are exactly 28 days. Usually, the cycles vary the most and the intervals between periods are longest in the years immediately after menarche and before menopause.

Menstrual bleeding lasts 3 to 7 days, averaging 5 days. Blood loss during a cycle usually ranges from ½ to 2½ 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.

The menstrual cycle is regulated by hormones. Luteinizing hormone and follicle-stimulating hormone, which are produced by the pituitary gland, promote ovulation and stimulate the ovaries to produce estrogen and progesterone. Estrogen and progesterone stimulate the uterus and breasts to prepare for possible fertilization. The cycle has three phases: follicular (before release of the egg), ovulatory (egg release), and luteal (after egg release).

Changes During the Menstrual Cycle

The menstrual cycle is regulated by the complex interaction of hormones: luteinizing hormone, follicle-stimulating hormone, and the female sex hormones estrogen and progesterone. The menstrual cycle begins with menstrual bleeding (menstruation), which marks the first day of the follicular phase. Bleeding occurs after estrogen and progesterone levels decrease at the end of the previous cycle. This decrease causes the top layers of thickened lining of the uterus (endometrium) to break down and be shed. About this time, the follicle-stimulating hormone level increases slightly, stimulating the development of several ovarian follicles. Each follicle contains an egg. Later, as the follicle-stimulating hormone level decreases, only one follicle continues to develop. This follicle produces estrogen. The ovulatory phase begins with a surge in luteinizing hormone and follicle-stimulating hormone levels. Luteinizing hormone stimulates egg release (ovulation), which usually occurs 16 to 32 hours after the surge begins. The estrogen level peaks during the surge, and the progesterone level starts to increase. During the luteal phase, luteinizing hormone and follicle-stimulating hormone levels decrease. The ruptured follicle closes after releasing the egg and forms a corpus luteum, which produces progester-one. During most of this phase, the estrogen level is high. Progesterone and estrogen cause the lining of the uterus to thicken more, to prepare for possible fertilization. If the egg is not fertilized, the corpus luteum degenerates and no longer produces progesterone, the estrogen level decreases, the top layers of the lining break down and are shed, and a new menstrual cycle begins.

Follicular Phase: This phase begins on the first day of menstrual bleeding (day 1). But the main event in this phase is the development of follicles in the ovaries.

At the beginning of the follicular phase, the lining of the uterus (endometrium) is thick with fluids and nutrients designed to nourish an embryo. If no egg has been fertilized, estrogen and progesterone levels are low. As a result, the top layers of the endometrium are shed, and menstrual bleeding occurs.

About this time, the pituitary gland slightly increases its production of follicle-stimulating hormone. This hormone then stimulates the growth of 3 to 30 follicles. Each follicle contains an egg. Later in the phase, as the level of this hormone decreases, only one of these follicles (called the dominant follicle) continues to grow. It soon begins to produce estrogen, and the other stimulated follicles begin to break down.

On average, the follicular phase lasts about 13 or 14 days. Of the three phases, this phase varies the most in length. It tends to become shorter near menopause. This phase ends when the level of luteinizing hormone increases dramatically (surges). The surge results in release of the egg (ovulation).

Ovulatory Phase: This phase begins when the level of luteinizing hormone surges. Luteinizing hormone stimulates the dominant follicle to bulge from the surface of the ovary and finally rupture, releasing the egg. The level of follicle-stimulating hormone increases to a lesser degree. The function of the increase in follicle-stimulating hormone is not understood

The ovulatory phase usually lasts 16 to 32 hours. It ends when the egg is released.

About 12 to 24 hours after the egg is released, the surge in luteinizing hormone can be detected by measuring the level of this hormone in urine. This measurement can be used to determine when women are fertile. The egg can be fertilized for only 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.

Luteal Phase: This phase begins after ovulation. It lasts about 14 days (unless fertilization occurs) and ends just before a menstrual period. In this phase, the ruptured follicle closes after releasing the egg and forms a structure called a corpus luteum, which produces increasing quantities of progesterone. The corpus luteum prepares the uterus in case fertilization occurs. The progesterone produced by the corpus luteum causes the endometrium to thicken, filling with fluids and nutrients to nourish a potential fetus. Progesterone causes the mucus in the cervix to thicken, so that sperm or bacteria are less likely to enter the uterus. 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 Infertility: Problems With Ovulation). During most of the luteal phase, the estrogen level is high. Estrogen also stimulates the endometrium to thicken.

The increase in estrogen and progesterone levels causes milk ducts in the breasts to widen (dilate). 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.

Puberty is a sequence of events in which physical changes occur, resulting in adult physical characteristics and capacity to reproduce. These physical changes are regulated by changes in the levels of hormones that are produced by the pituitary gland—luteinizing hormone and follicle-stimulating hormone. At birth, levels of these hormones are high, but they decrease within a few months and remain low until puberty. Early in puberty, levels of luteinizing hormone and follicle-stimulating hormone increase, stimulating the production of sex hormones. The increased levels of sex hormones (primarily estrogen) result in physical changes, including maturation of the breasts, ovaries, uterus, and vagina. Normally, these changes occur sequentially during puberty, resulting in sexual maturity (see Adolescents:Physical and Sexual Development).

The first change of puberty is usually the start of breast development (breast budding). In girls who live in the United States, this change usually occurs around age 8 to 13. Shortly afterward, pubic and underarm hair begin to grow. The interval from breast budding to the first menstrual period is usually about 2½ years. In the United States, girls, on average, have their first period when they are almost 13. The girl's body shape changes, and the percentage of body fat increases.

The growth spurt accompanying puberty typically begins about when pubic and underarm hair begin to grow. Growth is fastest relatively early in puberty (before menstrual periods begin) and peaks at about age 12. Then growth slows considerably, usually stopping between the ages of 14 and 16.