Age of Menarche
enarche is the "hallmark maturation event" in young girls (Towne et al., 2005). In the United States, the average age of first menarche is 12.7 years, but the age has been steadily decreasing each decade (Remsberg et al., 2005). One hundred years ago, the average age of first menstruation was about 16 years old. Over the last 20 to 30 years, however, decreases in age of first menarche have been small (Whincup et al.,
2001) . In other, less-developed countries, the age of menarche is later. For example, in rural Chile, the average age of first menarche is close to 14 years old (Dittmar, 2000). Environmental factors, such as high altitudes and poor nutrition, can delay the age at which a girl begins menstruating. In fact, girls with poor nutrition and with substandard living conditions begin menstruating later than girls with either adequate nutrition or adequate living standards (Dittmar, 2000). It is hard to say exactly why this is. There is also a heritability component to the age at which a woman menstruates—many girls reach menarche and menopause
at approximately the same age as their biological mothers (Towne et al., 2005).
Severe exercise regimens, such as long distance running or intense ballet dancing, may delay puberty in young girls. The onset of puberty is triggered by the acquisition of a certain body weight and appropriate fat-to-muscle ratio (Warren et al., 2002). Research has also found that family stress may be associated with an earlier entrance into puberty for girls (K. Kim & Smith, 1999; Ravert & Martin,
1997) . Moderate to high levels of stress tend to stimulate early maturation in girls.
There is some evidence to suggest that girls who reach menarche earlier may be at risk for developing physical problems, including high blood pressure and/or glucose intolerance (Remsberg et al., 2005). In addition, those girls who reached menarche before the age of 11 have been found to be more "norm breaking," having earlier sexual experiences, less academic education in adulthood, and less body satisfaction (Johansson & Ritzen, 2005).
the follicles in the ovaries begin to ripen with the next cycle’s ova, estrogen released by the ovaries stimulates regrowth of the endometrium’s outer layer, to about 2 to 5 millimeters thick.
During the ovulation phase, an ovum is released, usually about the 14th day of the cycle. The particulars of ovulation were described in the preceding section on the ovaries and Fallopian tubes. The third phase is the luteal phase. Immediately following ovulation, a small, pouchlike gland, the corpus luteum, forms on the ovary. The corpus luteum secretes additional progesterone and estrogen for 10 to 12 days, which causes further growth of the cells in the endometrium and increases the blood supply to the lining of the uterus. The endometrium reaches a thickness of 4 to 6 millimeters during this stage (about a quarter of an inch) in preparation to receive and nourish a fertilized egg.
Question: When I started college, I began having very severe menstrual cramps—often to the point of nausea, fatigue, and backache. What causes bad cramps, and how can I reduce them?
Menstrual cramps are usually caused by prostaglandins, which stimulate the uterus to contract and expel the endometrial lining during menstruation. The uterine muscles are powerful (remember that the muscles help push an infant out at birth), and the menstrual contractions can be strong and are sometimes quite painful. However, there are many things that can make the cramps worse. Poor eating habits, an increase in stress, alcohol use, insufficient sleep, and a lack of exercise can aggravate the problem. Reducing salt, sugar, and caffeine intake; moderate exercise; warm baths; and gentle massage of the lower back sometimes help, as do antiprostaglandin pain relievers, such as ibuprofen. Orgasm, either through masturbation or with a partner, also helps many women relieve menstrual cramps.
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Figure 4.5
The ovarian and menstrual cycles.
If fertilization does not occur, however, the high levels of progesterone and estrogen signal the hypothalamus to decrease LH and other hormone production. The corpus luteum begins to degenerate as LH levels decline. Approximately 2 days before the end of the normal cycle, the secretion of estrogen and progesterone decreases sharply as the corpus luteum becomes inactive, and the menstrual stage begins.
In the menstrual phase, the endometrial cells shrink and slough off (this flow is referred to as menses (MEN-seez). The uterus begins to contract in an effort to expel the dead tissue along with a small quantity of blood (it is these contractions that cause menstrual cramps, which can be painful in some women). During menstruation, approximately 35 milliliters of blood, 35 milliliters of fluid, some mucus, and the lining of the uterus (about 2 to 4 tablespoons of fluid in all) are expelled from the uterine cavity through the cervical os and ultimately the vagina. (If a woman is using oral contraceptives, the amount may be significantly smaller; see Chapter 13.) Some women lose too
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Hypothalamus
GnRH
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Estrogen & progesterone decrease release of GnRH
much blood during their menstruation and may develop anemia. Menses usually stops about 3 to 7 days after the onset of menstruation.
This monthly cyclical process involves a negative feedback loop, in which one set of hormones controls the production of another set, which in turn controls the first (see Figure 4.6). In women, the negative feedback loop works like this: Estrogen and progesterone are produced by the ovaries at different levels during different parts of the menstrual cycle. As these levels increase, the hypothalamus is stimulated to decrease its production of GnRH, which sends a message to the pituitary to decrease levels of FSH and LH. The decrease in FSH and LH signals the ovaries to decrease their production of estrogen and progesterone, so the hypothalamus increases its level of GnRH, and it all begins again. This process is similar to a thermostat; when temperatures go down, the thermostat kicks on and raises the temperature, until the rising heat turns off the thermostat and the heat begins slowly to fall.
ercise, eating disorders, emotional factors, certain diseases, surgical removal of the ovaries or uterus, or hormonal imbalance caused naturally or through the ingestion of steroids. For example, almost all women with anorexia nervosa will experience amenorrhea. When they regain weight, they often will not begin ovulating and menstruating and may need drugs to induce ovulation in order to start their periods again (Biro et al., 2003). If amenorrhea persists, a physician should be consulted.
Question: Someone once told me that women who live together often experience menstruation at the same time. Why does this happen?
Menstrual synchronicity, as this phenomenon is called, is common, and women who live in the same apartment or house often notice that they begin to cycle together (however, this will only happen if the women are not using hormonal forms of birth control). Menstrual synchronicity occurs because of pheromones, chemicals that are produced by females (more powerfully in animals) during their fertile periods that signal their reproductive readiness. Women who live together detect each other’s pheromones (unconsciously), and slowly their fertile periods begin to converge.
Some women suffer from menorrhagia (men-or-RAY-gee-uh), or excessive menstrual flow. Often oral contraceptives are prescribed to make menses lighter and more regular. Later in this chapter we will discuss some newer options that women have to avoid menstruation altogether.
Dysfunctional uterine bleeding (DUB), when a woman bleeds for long periods of time or intermittently bleeds throughout her cycle, is another common disorder. DUB is usually caused by conditions such as hormonal imbalance, significant weight loss, eating disorders, stress, chronic illness, and excessive exercise (Oriel & Schrager, 1999). A woman who bleeds throughout her menstrual cycle should see her healthcare provider. Untreated DUB can lead to medical problems, such as anemia, and can also cause social embarrassment, because some women need to change their sanitary pads and tampons as often as once an hour.
Dysmenorrhea (dis-men-uh-REE-uh), or painful menstruation, may be caused by a variety of inflammations, by constipation, or by psychological stress. In the past there was a tendency to believe that cramps were always the result of an organic problem, and some women even had operations in an attempt to stop the pain, but such strategies usually failed. Today, doctors recommend medication, relaxation, stress relief, yoga, and massage, all of which can bring some relief from dysmenorrhea.