About the Author
MIRIAM NELSON, PhD, is director of the Center for Physical Activity and Nutrition and associate professor of nutrition at Tufts University.
JENNIFER ACKERMAN is a science and health writer whose most recent book, Sex Sleep Eat Drink Dream, was named a New York Times Editors Choice.
Excerpt. © Reprinted by permission. All rights reserved.
1
Reproduction
The female reproductive system is an intricate and complex set of organs that carry out an amazing variety of tasks, from producing sex hormones to nurturing the miracle of new life. They include internal organs--ovaries, fallopian tubes, uterus, vagina, and accessory glands--and also the vulva, which covers the opening to the vagina. A woman's main reproductive organs are two ovaries--each about the size and shape of an almond--located on either side of the uterus. Ovaries produce eggs (ova) and sex hormones. Over the course of a lifetime, they produce, store, and release about 450 eggs in a process known as ovulation.
Two slender 4-inch fallopian tubes, or oviducts, connect the ovaries with the uterus. The end of each tube near the ovary is funnel shaped and fringed with fingerlike extensions called fimbriae that draw the egg into the tube. When an egg is released from your ovary, the fimbriae catch it and help push it along the fallopian tube in its 7-day journey to the uterus. Because the egg is only fertile for about a day, fertilization occurs in the fallopian tubes before the egg moves to the uterus.
Your uterus, or womb, provides the fertilized egg with a nurturing, hospitable environment in which to grow. The uterus is a powerful, muscular organ, normally about the size and shape of an upside-down pear. Its 1-inch- thick muscular walls can expand to accommodate a full-term fetus and help push the baby out during labor. The lining of the uterus, known as the endometrium, is where a fertilized egg arriving from the fallopian tube embeds and develops. If the egg is not fertilized, it dries up and, roughly 2 weeks later, exits the body along with menstrual flow consisting of sloughed tissue from the endometrium.
Your uterus opens to your vagina at the cervix, a strong, thick-walled opening normally no wider than a straw but capable of expanding to allow the passage of a baby. Within the cervix are glands that secrete mucus. This mucus varies in consistency from tacky and sticky to thin and clear and either assists or impedes sperm, depending on the time of your cycle.
From the cervix, the vagina runs about 4 inches to the vaginal opening. A hollow, accordion-like muscular tube lined with mucous membranes that keep it moist, the vagina is where the erect penis is inserted during sexual intercourse. It also serves as the birth canal and as a passageway for menstrual flow from the uterus. The vagina expands during sexual arousal and, especially, during childbirth. The lower third of the vagina is laced with many nerve endings and includes the Grafenberg spot, or G-spot, a sensitive spot roughly the size of a dime, 2 to 3 inches up just past the pubic bone; for some women, it is an area of erotic sensitivity. During sexual arousal, small glands on either side of the vagina known as Bartholin's glands may also swell and lubricate the passage. The opening to the vagina is known as the introitus; at birth it may be partially covered with a membrane of tissue called the hymen. It was once thought that a torn hymen was evidence of sexual intercourse, but that's simply not true. A hymen can be easily stretched, abraded, or torn by physical activity, use of tampons, masturbation, and other activities.
Your external genitalia, also known as your vulva or pudendum, include the mons pubis, the fleshy area just above your vaginal opening; the labia majora and labia minora, the two skin flaps surrounding the vaginal opening, which help keep bacteria out of the vestibule of the vagina; and the clitoris, a highly sensitive structure rich in blood supply and nerves, which swells during sexual arousal. Your clitoris is the only part of your body that is designed solely for pleasure.
While your breasts are not strictly necessary for procreation, they are part of your reproductive system and are sensitive to female hormones. Each breast has a raised nipple surrounded by a circular pigmented area called the areola, which contains muscles that make your nipple stand erect in response to touch and, sometimes, to cold. Your nipples contain openings for milk ducts within the breast. Inside, your breasts have lobes of glandular tissue (known as mammary glands) that include the sacs and tubes that make milk. The lobes are separated by protective fat and supported by connective tissue. The shape of your breast is determined by the amount and distribution of fat. The function of your mammary glands is regulated by estrogen and progesterone from your ovaries and, from your brain, prolactin and oxytocin--two hormones involved in breast development and milk production, among other things.
Female Reproductive System
The main organs of the female reproductive system include two ovaries, a pair of fallopian tubes (capped by fimbriae), and the uterus. An ovary releases the egg, which is swept by the fimbriae into the opening of the fallopian tubes, and from there, travels into the uterus.
Although we tend not to think of it this way, the brain is a powerful sexual organ, integral to both reproductive and sexual life. The pituitary gland, for instance, a structure about the size of a pea located just beneath the hypothalamus at the base of the brain, sends signals to the ovaries to prepare your eggs for ovulation. Both the hypothalamus and the pituitary gland play an important role in regulating female hormones.
YOUR REPRODUCTIVE CYCLES
A finely tuned array of interacting sex hormones orchestrates your reproductive cycles. At puberty, the pituitary gland in the brain begins to secrete two key hormones: folliclestimulating hormone (FSH) and luteinizing hormone (LH). These hormones stimulate your ovaries to make other hormones, including estrogen. Toward the end of puberty, your ovaries begin to release eggs--one per month--as part of your monthly menstrual cycle. The cycle has four phases:
1. Follicular. This phase begins just after menstruation ends and lasts for 6 to 13 days. The pituitary releases FSH and LH, stimulating the growth of a group of egg follicles, only one of which will eventually make a mature egg. Estrogen promotes the thickening of the endometrium in preparation for a fertilized egg.
2. Ovulatory. On around day 14 of your cycle, a mature egg is released into the fallopian tube--the process called ovulation. At this time of the cycle, your cervical mucus may become clear, copious, and stretchy, a state hospitable to sperm. The cervix opens a little.
3. Luteal. In this stage, progesterone and estrogen further stimulate the development of the endometrium. If there's no fertilization, however, the hormone levels drop. At this phase, the endometrium may produce prostaglandins--hormonelike substances that can trigger the cramps, breast tenderness, and mood swings of premenstrual syndrome.
4. Menstrual. The endometrial buildup, about 2 to 6 tablespoonfuls of blood and tissue, is expelled out of the uterus by uterine contractions. Normal menstrual flow can be light or heavy, regular or irregular, and can last from 3 to 7 days. After this, the endometrium rebuilds itself, and the cycle begins anew.
A girl's first period, called menarche, may occur anytime between the ages of 9 and 16. Although there are few statistics, it's widely believed that the age of menarche decreased by 2 to 3 years between 1900 and 1970, most likely due to better nutrition and health care. Today, some 10 percent of American girls reach menarche by age 11 and 90 percent by age 13.75. The average age of menarche in healthy American girls is 12.5, but it's perfectly normal to start menstruating at either end of the age spectrum. I didn't get my period until I was 16. At the time, I thought I was abnormal because all of my friends had already begun menstruating. But now I realize I was just at the older end of the age range. Once menarche takes place, most young women will have reached a height within an inch or two of their adult height. Some girls develop body image issues at menarche and during puberty. (This mental health issue is discussed in Chapter 23.)
After menarche, it can take up to 2 years or even more for a young woman to establish regular menstrual cycles. The typical menstrual cycle ranges from 20 to 40 days, with an average of about 28 days. However, there is great variation here, too. Some women experience irregular cycles for much of their premenopausal lives. I had very irregular periods until I got pregnant at the age of 27. Sometimes menstrual irregularities are due to hormone imbalances; consulting a physician can help clarify this.
Women's Menstrual Cycle
A woman's menstrual cycle includes a range of interacting events that prepare her body for pregnancy each month. The endometrium thickens; the ovary releases an egg in response to messages sent by the gonadrotropic hormones LH and FSH and the ovarian hormone estrogen. Rising progesterone levels stimulate the building up of the endometrium to provide a healthy environment for a fertilized egg to implant. If fertilization and implantation do not occur, progesterone levels drop, and the endometrium is sloughed off during menstruation. Body temperature rises just after ovulation and stays higher by about 0.4°F for 5 to 10 days, until menstruation.
A woman's reproductive cycles continue from menarche to menopause, when hormone levels change and reproductive cycles halt--usually in the late forties or early fifties. Cycles often get shorter first and then are erratic. But the pattern varies: Each woman's body follows its own script.
THE FEMALE ATHLETIC TRIAD
The link between disordered eating, amenorrhea, and low bone density
When I was a new graduate student in nutrition at Tufts University in 1984, my first research study explored the link between amenorrhea and bone density. Two previous studies had shown that young women who were amenorrheic had lower bone density--probably because of low estrogen levels. To find out more about the link, we recruited a group of young athletic women, half of whom had regular menstrual cycles and half of whom were amenorrheic. Because the laboratory I worked in was part of a nutrition research center, we were interested in finding out whether there were any nutrition issues involved in the association.
The results were surprising. As in the earlier studies, we found that the women who were amenorrheic had lower bone density despite being similar to the other women in weight and activity levels. But our nutrition studies also uncovered something new: The amenorrheic women had disordered eating habits. These women reported eating considerably fewer calories than the women with normal menstrual cycles, despite being similar in weight. In addition, about two-thirds of them were not getting enough protein in their diets. They were also engaging in some atypical eating patterns, skipping meals or eating very tiny meals 20 or 30 times a day. It appeared that this disordered eating and/or calorie restriction, not body weight or abundant exercise, was causing their amenorrhea.
When we published the study in the American Journal of Clinical Nutrition in 1987, it made national headlines and spurred a flurry of follow-up studies. The concept was later labeled the Female Athletic Triad.
Since then, much has been learned about the triad. We now know that amenorrhea among athletes is not normal--and not healthy. It increases their risk of infertility and also of stress fractures when they're young and osteoporosis in their later years. In the past, many female athletes thought it was convenient to be amenorrheic. But we now know that amenorrhea results in lower levels of estrogen and other hormones, and this is not good for bones. We also know that the cessation of menstrual periods among many (though not all) athletic women is most often caused not by high volume of exercise but by calorie restriction or disordered eating. Women who engage in rigorous exercise will most likely maintain regular menstrual cycles as long as they hold a stable body weight and eat well.
If you or someone you care about has irregular or absent menstrual periods, seek out a health care provider who can help you determine the root cause of the problem. You may want to start by consulting an endocrinologist. If an eating disorder is present, seek psychological help. Eating disorders are a mental health issue and should be addressed by a mental health expert. If there are serious nutritional deficiencies, you may also need to consult a nutritionist.
DISORDERS OF THE MENSTRUAL CYCLE
Menstrual patterns normally change and vary over the course of a lifetime. Flow may shift from light to heavy; monthly cycles may shorten or lengthen. In adolescence and again during perimenopause, women may experience irregular bleeding. After the age of 35, cycles often shorten. However, some menstrual irregularities such as the absence of periods or infrequent, prolonged, or heavy periods may reflect underlying disorders and should be checked out by a women's health clinician.
Among the common disorders of the menstrual cycle are:
* Amenorrhea. Amenorrhea, or the absence of menstruation in a premenopausal woman who is not pregnant, can occur during puberty or later in life. It can be a serious condition that may affect fertility and bone health. (See the box The Female Athletic Triad below.) Primary amenorrhea is not beginning menstruation by age 16. It may be caused by chromosomal abnormalities, problems with the hypothalamus or pituitary gland, structural abnormalities in the reproductive system, or anorexia. Secondary amenorrhea is missing several periods in a row once you have gone through menarche. Amenorrhea can be a normal result of breastfeeding or an intended effect of some kinds of birth control pills or the progesterone-containing IUD. (In these cases, it has none of the implications for fertility and bone health characterized by true secondary amenorrhea.) Or, it can be caused by stress, certain medications such as antidepressants and antipsychotics, polycystic ovary syndrome (PCOS), eating disorders such as anorexia or bulimia, low body weight, thyroid malfunction, premature menopause, and other disorders.
You should consult your health care provider if:
* You're age 16 or older and you've never had a menstrual period
* You've begun menstruating but have missed three or more consecutive periods
* Premenstrual syndrome (PMS). Roughly three-quarters of menstruating women experience some kind of premenstrual symptoms, ranging from mood changes to bloating and breast tenderness, especially from their late twenties to early forties. For some women, these symptoms may be so severe that they interfere with daily life. This condition is called premenstrual syndrome. Among the symptoms are severe irritability, tension, anxiety, mood swings, or difficulty concentrating, as well as pronounced physical symptoms such as breast tenderness, abdominal bloating, fatigue, acne, food cravings, and increased appetite. Symptoms most often occur in the second half of the menstrual cycle and resolve within a few days of the onset of menses.
For relief or control of mild PMS symptoms, you can try making lifestyle changes:
* Get aerobic exercise--at least 30 minutes three times a week.
* Reduce stress through relaxation techniques such as yoga and/or deep breathing.
* Get adequate sleep.
* Take a nonsteroidal anti-inflammatory drug (NSAID) such as ibuprofen.
