Menstruation is a woman's monthly discharge of the inner lining of the uterus (endometrium). It lasts about 4 to 5 days and
occurs, on the average, every 28 days. During the menstrual cycle, an egg (oocyte) matures and is released, a process called
ovulation. At the same time, the uterus lining thickens to create a suitable environment in which a fertilized egg could develop into a fetus. If fertilization does not occur, this lining is shed, resulting in the menstrual period. Thus, failure to menstruate can be the first indication of pregnancy.
PHASES OF THE CYCLE
The first day of the menses, or the onset of the menstrual flow, is considered the first day of the cycle. Although 28 days is considered an average cycle, the time may range from 21 to 35 days. Climate, emotional factors, age, and drugs can alter the length of the menstrual cycle.
Days 1 to 13 of the cycle are called the follicular phase. This is the time of growth for the follicles, the small, round structures that surround each egg in the ovary. Women are born with about 2 million follicles. At the start of each menstrual cycle a group of follicles enlarges and fills with fluid. By the fifth or sixth day of the cycle, one of them begins to grow toward an ultimate size of about 2 cm (.79 in), while the others disintegrate. At midcycle, day 14, the oocyte within this follicle matures, the follicle ruptures and expels the oocyte, and ovulation occurs.
The oocyte is released into the body cavity where it is picked up by the fimbria, fringelike structures at the end of the Fallopian tubes, or oviducts, that envelop each ovary. The fimbria direct the oocyte into the Fallopian tubes, propelling it to the ampulla, the region of the Fallopian tubes where fertilization occurs. Fertilization usually takes place within 24 hours of ovulation, and the embryo is implanted in the uterus about six days later. If fertilization does not occur, the oocyte degenerates and is reabsorbed by normal bodily processes.
Days 14 to 28 make up the second part of the cycle, called the luteal phase, because, following ovulation, the collapsed follicle transforms into a corpus luteum and secretes
progesterone. The luteal phase is the most constant part of the normal menstrual cycle. In contrast, the follicular phase is more variable.
HORMONAL CONTROL OF THE CYCLE
The menstrual cycle is under strict hormonal control. At the beginning of the follicular phase there is an increase in the secretion of follicle-stimulating hormone (FSH) from the pituitary gland, located at the base of the brain. This rise in FSH stimulates the growth of a dominant follicle.
As the pituitary gland releases more FSH, the dominant follicle secretes more estrogen. Estrogen causes the uterine glands to grow and the lining of the uterus to thicken in preparation for implantation of the embryo. However, the increased levels of estrogen also inhibit the FSH-releasing center of the brain, resulting in a decrease in the secretion of FSH. The increasing levels of estrogen also trigger the brain's releasing center for luteinizing hormone (LH), causing a midcycle (day 14) LH discharge from the pituitary. Ovulation occurs 36 hours after onset of the LH surge.
After ovulation, LH causes the cells of the ruptured follicle to undergo a transformation into the corpus luteum. The corpus luteum still secretes estrogen, but a smaller quantity than was secreted prior to ovulation. More importantly, the corpus luteum secretes the second major female sex hormone, progesterone. During the luteal phase, progesterone causes the glands in the uterine lining to mature and to begin to secrete substances that are essential for the survival and implantation of the embryo. Progesterone also inhibits the FSH-releasing centers in the brain, preventing the release of FSH and the start of a new cycle.
MENSES
If fertilization does not occur, the elevated levels of progesterone secreted by the corpus luteum inhibit the LH-releasing cters in the brain, causing a decline in the level of LH. Because LH is responsible for maintaining the secretion of progesterone from the corpus luteum, the corpus luteum begins to shrink and stops secreting progesterone. The corpus luteum degenerates, and its hormone production ceases; the FSH-releasing mechanisms in the brain are freed from their inhibition, and FSH levels begin to rise, signaling the start of a new menstrual cycle.
Meanwhile, as the progesterone levels decline the hormonal support to the uterine lining is lost, and the body begins to reabsorb the tissue that was built up during the cycle. More tissue is present than can be reabsorbed, and this is the monthly menstrual flow, which consists of this excess uterine epithelium as well as blood. Menstruation lasts about 4 to 5 days.
PHYSICAL MANIFESTATIONS OF THE CYCLE
The cyclic appearance of a large collection of physical and behavioral symptoms has been recognized as premenstrual syndrome (PMS). The most frequent physical symptoms include abdominal bloating and cramping, breast tenderness, fluid retention, and headaches. Behavioral symptoms include anxiety, crying spells, depression, fatigue, irritability, and appetite changes. These symptoms usually occur in the last 7 to 10 days of the cycle. Although the exact cause of PMS is unknown, it is thought to be due to changing hormonal levels.