The Menstrual Cycle

The Menstrual Cycle

The onset of menstruation or the time of the first menstrual period is usually accepted as being the date at which puberty occurs. This, however, is not strictly true. Puberty represents very extensive changes that occur throughout the whole body, and the process is really the changing from a child into a woman. The completion of this process may take as long as ten years and at some stage during these ten years menstruation begins.

The menarche is the technical term for the date of onset of the first period. This varies in different races and in different parts of the world, but generally occurs earlier in hotter climates than it does in temperate zones. Nevertheless, the age of the menarche is gradually becoming younger and in Great Britain today it usually occurs between the ages of twelve and fourteen, although it is not uncommon at ten or even nine.

The Menstrual CycleThe first changes of puberty may be recognized as early as six or seven years and result from the initial secretions of oestrogen from the maturing ovaries. The earliest features that may be noticed are subtle changes in body contour together with very early development of the breasts. These so-called secondary sexual characteristics continue gradually and slowly until the fully mature woman is developed. This is usually at the age of eighteen whereas in the male development may continue for some time longer.

The whole development of the mature female is a very gradual and carefully planned process. The secondary sexual characteristics such as the development of the breasts, the distribution of fat over the body, the growth and distribution of hair, proceed in a very carefully organized and logical manner. Development of the sexual organs, namely the vulva, the clitoris, the vagina, the uterus and the Fallopian tubes and maturation of the ovaries proceed at the same time, and it is during this time that the first menstrual period occurs.

The onset of menstruation is usually unexpected, but may be accompanied by premonitory signs and symptoms. The establishing of a normal regular menstrual cycle may take months or even years, and while a large proportion of girls do in fact have a normal menstrual rhythm from the start, it is not abnormal if it takes several years to establish a regular cycle. Irregular bleeding at this time does not require any treatment, but prolonged or excessive menstrual loss should be considered abnormal and medical advice sought.

The normal menstrual cycle is represented by bleeding which occurs from the uterus every 28 days. In order to understand the menstrual cycle, however, it is necessary to understand some of the factors that control menstruation. There are three basic cycles involved.

1. The Pituitary Cycle

The pituitary controls the menstrual cycle by secreting follicle stimulating hormone which provokes the formation of many follicles within each ovary. The developing follicles secrete oestrogen and continue to develop in competition until finally one follicle alone reaches maturity (occasionally more in the case of a multiple pregnancy) on the 14th day of the cycle. As soon as this happens the other competing follicles atrophy so that only one ovum is normally released during each menstrual cycle. The pituitary gland secretes luteinizing hormone which causes ovulation and stimulates formation of a corpus luteum from the ruptured follicle. This continues for the next 12 days until the 26th day of the menstrual cycle, when, in the absence of pregnancy, the production of luteinizing hormone ceases and the corpus luteum degenerates. This is followed on the 28th day by the onset of menstruation and thereafter the whole cycle begins again with the secretion of the follicle stimulating hormone.

2. The Ovarian Cycle

Under the stimulus of follicle stimulating hormone from the pituitary glands, follicles commence to ripen in both ovaries. These are tiny cysts, each one of which is capable of producing an ovum ready for fertilization. Only one follicle, however, generally reaches maturity and it does so on the 14th day of the cycle. At the beginning of the menstrual cycle perhaps one hundred or more follicles in each ovary commence to ripen. They produce and secrete oestrogen increasingly as they ripen, and the production reaches its peak at the time of ovulation. The follicle which has been selected for final ripening matures very rapidly and distends the surface of the ovary as a cystic swelling almost half an inch in diameter. On the 14th day it ruptures and discharges the fully mature ovum into the outer end of the Fallopian tube. The follicular fluid which contains a large amount of oestrogen is absorbed. The other competing follicles atrophy and degenerate. It is not known why generally only one follicle is selected when many commence to mature. Some of the so-called ‘fertility drugs’ contain a follicle stimulating hormone and if this is administered either to a woman who is unduly sensitive or in extra large doses, then more than one follicle may ripen or alternatively one follicle may ripen from each ovary, and it is easy to understand how multiple pregnancies can result.

Just before ovulation the pituitary gland secretes luteinizing hormone, which causes ovulation and the formation of the corpus luteum from the newly ruptured follicle. The corpus luteum secretes progesterone. If fertilization does not occur the corpus luteum begins to degenerate on the 26th day of the menstrual cycle with the resulting fall in progesterone production and subsequent menstruation.

Thus the ovarian cycle results first in the production of oestrogen, then in ovulation and finally in the production of both oestrogen and progesterone which fall immediately prior to menstruation.

3. The Uterine Cycle

The menstrual cycle normally lasts for 28 days, although regular variations between 21 and 35 days or even longer are considered normal. The first day of the menstrual period is the first day of the menstrual cycle. Bleeding occurs from the uterus as a result of a breakdown and degeneration of the thick and luxuriant lining of the cavity of the uterus. This breakdown is a direct result of the fall in levels of oestrogen and progesterone on the 26th day of the preceding menstrual cycle. If fertilization has not occurred it is necessary for the body to produce another ovum for fertilization. The lining of the uterus that has been hopefully prepared for a pregnancy during the previous month is shed in order that a new lining can be prepared to be ready to receive a fertilized ovum should pregnancy occur during the next month. The superficial layers of the lining of the endometrium disintegrate accompanied by a certain amount of bleeding. The disintegration and shedding of the endometrium takes about three days and thereafter the amount of blood lost during the period gradually diminishes as the endometrium heals.

Following the end of the period the action of oestrogen provokes the regeneration of the endometrium in what is known as the proliferative phase. This results in the rapid growth of a thick and normal lining to the uterine cavity. When ovulation has occurred and the secretion of progesterone begins, the endometrium under¬goes what is known as secretory change. This change, directly under the influence of progesterone, is vital for the nutrition of a newly fertilized ovum. The secretory change commences on the 15 th day and continues until the 26th day of the cycle. The glands in the endometrium enlarge and secrete material for the nutrition of the new ovum.

Most women rapidly establish what is known as a normal menstrual pattern in which not only is their menstrual cycle regulated but also the number of days during which bleeding occurs, together with the pattern of the bleeding. Each woman establishes her own menstrual pattern and while the majority menstruate every 28 days, it is perfectly normal for a woman to menstruate as frequently as every 21 days, or as infrequently as every 3 months. Equally there may be a variation in the duration of bleeding from 2 days to 8 days, as well as a variation in the actual amount of blood lost during a period.