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Puberty – The arrival of the monthly friend .
The principle sign that a girl has become sexually mature and is capable of reproduction is the onset of menstruation, a period of cyclic bleeding from the vagina. (Smith 1997, pg 217) This is one of the key physiological functions of the female body. An ovarian, or estrus, cycle is characteristic of all female mammals, but menstruation, periodic uterine bleeding that accompanies the ovarian cycle, occurs only in women, female apes, and some monkeys.
The average length of the menstrual cycle is twenty-eight days, although somewhat shorter and longer cycles are quite common. The day that bleeding starts is counted as the first day of a given cycle. The menstrual cycle is controlled by mechanisms involving hormones released by the hypothalamus, the pituitary, and the ovaries. The cycle has four stages: the menstrual phase, the preovulatory phase, the ovulation phase, and the postovulatory phase. Refer to Graph One.
The preovulatory phase starts as soon as menstrual bleeding from the previous cycle has ended. The anterior pituitary produces large amounts of follicle stimulating hormone (FSH) and a small amount of luteinizing hormone (LH). Under the influence of FSH, an ovarian follicle begins to mature and produces, in turn, increasingly higher levels of estrogens. In response to estrogen stimulation, the uterine lining thickens with increased numbers of blood vessels and uterine glands.
In the ovulation phase at midcycle, the ovum is released as LH production surges and FSH output decreases. Gonadotropic hormones that control the output of ovarian hormones are in turn regulated by the level of ovarian hormones through a negative feedback mechanism. Thus, the increased level of estrogens depresses FSH production. Following ovulation, the follicle transforms into the corpus luteum (”yellow body”) which produces progesterone. Around ovulation, some women feel a twinge or cramp in the lower abdomen or back, sometimes with vaginal discharge, perhaps bloody. This is called Mittelschmerz (”middle pain”). A few women have headaches, gastric pains, or sluggishness. Other women feel better around ovulation — it’s definitely an individualistic thing.
In the postovulatory phase, the secretion of progesterone begins to rise. Under the combined influence of estrogens and progesterone, the endometrium continues to develop and the uterine glands secrete nutrient materials. The endometrium is now ready to receive and sustain the fertilized ovum, if fertilization has occurred. The levels of ovarian hormones then remain high and the uterine lining is maintained intact through pregnancy. If pregnancy doesn’t occur, the high levels of estrogens and progesterone gradually fall.
Below a certain level of hormonal support, the uterine lining can no longer be maintained and begins to slough off, initiating the final menstrual phase of the cycle. As levels of ovarian hormones drop, their inhibiting effect on the pituitary gonadotropins is lifted. FSH and LH production now begins to rise, and a new cycle starts.
Women tend to ovulate mid-cycle; however, it is more accurate to say that they ovulate fourteen days before menstruation. Women have been known to ovulate at any time during their cycle, including during menstruation, although this is unusual. In terms of conception, fertility depends on three factors: a healthy egg, healthy sperm, and favorable cervical mucus. A woman ovulates once a cycle. The egg lives twelve to twenty-four hours and then disintegrates if not fertilized. Under favorable cervical mucus conditions (cervical mucus nourishes and guides the sperm, which would otherwise die in about a half-hour or never reach the egg), sperm can survive as long as five days within the body.
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