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The Merck Manual of Medical Information--Home Edition
Section 22. Women's Health Issues
Chapter 232

Hormones and Reproduction

Normal human reproduction involves the interaction of a variety of hormones and organs, orchestrated by the hypothalamus, an area of the brain. In both females and males, the hypothalamus secretes hormones, called releasing factors, that travel to the pituitary, a pea-sized gland located directly below the hypothalamus. (see illustration, page 697) These hormones stimulate the pituitary to release other hormones. For example, gonadotropin-releasing hormone, a releasing factor secreted by the hypothalamus, stimulates the pituitary to secrete luteinizing hormone and follicle-stimulating hormone. These hormones stimulate the reproductive glands to mature and to release sex hormones. The ovaries in women release estrogens, and the testes in men release androgens such as testosterone. Sex hormones are also produced by the adrenal glands, located on top of the kidneys.

The patterns of secretion and resulting blood levels of sex hormones determine whether these hormones stimulate or inhibit the release of luteinizing hormone and follicle-stimulating hormone from the pituitary. For example, a decrease in the sex hormone levels stimulates the pituitary to release larger amounts of the two hormones--a negative feedback control mechanism. Virtually all hormones are released in short bursts (pulses) every 1 to 3 hours. As a result, hormone levels in the bloodstream fluctuate.


At birth, the levels of luteinizing hormone and follicle-stimulating hormone are high, but they decrease within a few months and remain low until puberty. Early in puberty, these hormone levels increase, stimulating the production of sex hormones. In girls, the increased hormone levels stimulate the breasts, ovaries, uterus, and vagina to mature; menstrual periods to start; and secondary sexual characteristics--such as pubic and underarm hair--to develop. In boys, the testes, prostate, seminal vesicles, and penis mature, and facial, pubic, and underarm hair grows. Normally, these changes occur sequentially during puberty, resulting in sexual maturity. (see illustration, page 1256)

In girls, the first change of puberty usually is breast budding (the breasts start to develop), followed closely by the growth of pubic and underarm hair. The interval from breast budding to the first menstrual period generally is about 2 years. The girl's body shape changes, and the percentage of body fat increases. The growth spurt accompanying puberty typically begins even before the breasts start to develop. Growth is fastest relatively early in puberty, before menstrual periods begin. Then growth slows considerably, usually stopping between ages 14 and 16. In contrast, boys grow fastest between ages 13 and 17 and can continue to grow into their early 20s.

The age at which puberty begins seems to be influenced by a child's general health and nutrition as well as by socioeconomic and hereditary factors. In Western Europe, the average age at which a girl has her first menstrual period decreased by 4 months for each decade between 1850 and 1950, but the age hasn't decreased in the last four decades. Girls who are moderately obese tend to start menstruating earlier; girls who are severely underweight and malnourished tend to start later. Periods also start earlier among girls who live in urban areas and among those whose mothers started menstruating early.

Menstrual Cycle

Menstruation, the shedding of the lining of the uterus (the endometrium) accompanied by bleeding, occurs in approximately monthly cycles unless a woman is pregnant. It marks the reproductive years of a woman's life, extending from the start of menstruation (menarche) during puberty until its cessation (menopause).

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 percent of cycles are exactly 28 days. The intervals between periods are generally longest in the years immediately after menarche and before menopause. The menstrual cycle can be divided into three phases: follicular, ovulatory, and luteal.

The follicular phase, which varies in length, extends from the first day of bleeding to immediately before a surge in the level of luteinizing hormone, which causes the egg to be released (ovulation). This phase is so named because the follicles in the ovaries are developing during it. During the first half of the phase, the pituitary gland slightly increases its secretion of follicle-stimulating hormone, stimulating the growth of 3 to 30 follicles, each containing an egg. Only one of these follicles continues to grow. The other stimulated follicles degenerate. The follicular phase tends to become shorter at the end of the reproductive years, near menopause.

In a menstrual period, part of the endometrium is shed in response to a decrease in estrogen and progesterone levels. The endometrium consists of three layers. The top (superficial) layer and most of the middle (intermediate) layer are shed. The bottom (basal) layer remains and produces new cells to rebuild the other two layers. Menstrual bleeding lasts 3 to 7 days, averaging 5 days. Blood loss ranges from ½ to 10 ounces, averaging 4½ ounces. A sanitary pad or tampon, depending on the type, can hold up to an ounce. Menstrual blood usually doesn't clot unless the bleeding is very heavy.

How Many Eggs?

A baby girl is born with eggs (oocytes) already in her ovaries. By the time a female fetus is 20 to 24 weeks old, the fetus' ovaries contain 7 to 20 million eggs. The eggs become incorporated into follicles (fluid-filled cavities, each with an egg embedded in its wall). While the follicles are forming, most of the eggs gradually waste away, leaving about 2 million present at birth. No more develop after birth. Fewer than 400,000 remain by the time menstrual periods start--more than enough eggs for a lifetime of fertility.

Only about 400 eggs are released during a woman's reproductive life, usually one during each menstrual cycle. Until released, an egg remains dormant in its follicle--suspended in the middle of a cell division--making the egg one of the longest-lived cells in the body. Because the dormant egg can't perform the usual cellular repair processes, the opportunity for damage increases as a woman ages. A chromosomal or genetic abnormality is thus more likely when a woman conceives a baby later in life.

The ovulatory phase, during which the egg is released, starts with a surge in the level of luteinizing hormone. The egg is usually released 16 to 32 hours after the surge begins. The one follicle that's growing bulges out from the surface of the ovary, finally rupturing and releasing the egg. Around the time of ovulation, some women feel a dull pain on one side of the lower abdomen, known as mittelschmerz, which may last for a few minutes to a few hours. Although the pain is felt on the same side as the ovary that released the egg, the precise cause of the pain isn't known. The pain may precede or follow the rupture of the follicle and may not occur in all cycles. Egg release doesn't 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 corpus luteum, which secretes increasing quantities of progesterone.

click on thumbnail to see full-size imageThe progesterone causes body temperature to rise slightly during the luteal phase and remain elevated until a menstrual period begins. This rise in temperature can be used to estimate whether ovulation has occurred. (see page 1116 in Chapter 240, Infertility)

The corpus luteum degenerates after 14 days and a new menstrual cycle begins unless the egg is fertilized. If the egg is fertilized, the corpus luteum begins to produce human chorionic gonadotropin. This hormone maintains the corpus luteum, which produces progesterone, until the growing fetus can produce its own hormones. Pregnancy tests are based on detecting increased levels of human chorionic gonadotropin.

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