menstrual cycle

menstrual cycle Throughout a woman's reproductive life — from puberty to the menopause — the ovaries are programmed to produce a mature egg (ovum) approximately every 28 days and to prepare the uterus (womb) for implantation of an embryo if the egg becomes fertilized. To achieve this reproductive competence the ovaries must receive the correct hormonal signals from the brain and the pituitary gland. These signals stimulate the production of female sex hormones and the cyclical changes which occur in the ovary during each normal menstrual cycle. In turn the sex steroids released by the ovary induce changes in the lining of the womb and other parts of the female reproductive tract. The system is subtly regulated and fine-tuned by feedback effects of the ovarian steroid hormones on hormone secretions from the hypothalamus and pituitary gland, so there is a complex interplay of hormones and feedback signals which ultimately controls female fertility. Collectively these events constitute the menstrual cycle.

The first day of the menstrual cycle is defined as the first day of menstrual blood loss. This is when the uterus begins to shed its lining and bleeding occurs. At this time the secretion of hormones (oestrogen and progesterone) from the ovaries is at a minimum. This diminishes the braking effect that circulating ovarian hormones have on the secretion of the gonadotrophic hormones from the pituitary gland, namely luteinizing hormone (LH) amd follicle stimulating hormone (FSH). As a consequence these pituitary secretions increase and stimulate a new wave of activity in the ovaries.

Early in the cycle, FSH stimulates growth of a few follicles (egg-containing ‘sacs’) in each ovary. By about day 10 the ovaries contain several follicles with a diameter of 14–21 mm. As mid cycle approaches, all but one of these degenerate, and only the ‘dominant’ follicle becomes fully mature, with a diameter of 20–25 mm. What determines which follicle becomes the dominant one, and in which ovary, remains speculative. Local hormones or other factors acting within the ovaries may play an important role. This first half of the ovarian cycle is known as the follicular phase and is characterized by increasing secretions of oestrogen from the developing follicles; this is released into the bloodstream, reaches the uterus, and causes its lining to thicken: the glands enlarge and it becomes richly supplied with blood vessels: the proliferative phase of the uterine cycle.

In most normal human menstrual cycles only one follicle reaches full maturity, to be released at ovulation, on about day 14. The occasional release of two accounts for non-identical twins, and fertility drugs can increase the number of follicles reaching maturity at mid cycle. These drugs are either pituitary gonadotrophins, or synthetic chemicals which interfere with the negative feedback loop in such a way as to promote an increase in the release of these hormones from the pituitary gland itself. In both cases the ovaries receive an increased ‘drive’ for follicular development, and thus several follicles will mature. Such drugs are used for treating certain types of infertility, and are given to women undergoing in vitro fertilization (IVF) treatment. If the result is multiple ovulation, the chances of fertilization are increased or, in the case of IVF, more than one mature egg can be recovered for external fertilization and subsequent implantation.

At mid cycle there is a dramatic change of events. There is a high blood concentration of oestrogen, but this ceases to have a braking (negative feedback) effect on the pituitary hormones. About 24–48 hours after the peak of oestrogen production a surge of the gonadotrophins occurs — especially of luteinizing hormone. This is one of the rare biological examples of a positive feedback action. The surge causes the mature ‘dominant’ follicle to rupture and release its egg within 9–12 hours. Indeed, one way of predicting ovulation is by the detection of the increase in luteinizing hormone in the blood, which is reflected in the urine. This is the scientific basis for the kits which are commercially available to identify the most likely time for conception.

At the time of ovulation there is a small rise in body temperature. This is thought to be due to the action of rising progesterone in the blood, resetting in some way the ‘thermostat’ in the brain which controls our body temperature. This small rise can be used to indicate when ovulation occurs, but obtaining reliable temperature measurements is difficult, making the method often unsatisfactory. Some women feel mild pain in the abdomen around the time of ovulation, lasting from a few minutes to a couple of hours. Known as Mittelschmerz (German for ‘midpain’), it is probably caused by irritation of the abdominal wall due to blood and fluid escaping from the ruptured follicle. Changes in the cervical mucus also occur about the time of ovulation.

After ovulation the empty follicle left behind in the ovary is remodelled, and it plays an important role in the second half of the menstrual cycle, known as the luteal phase of the ovarian cycle. The cells remaining in the ruptured follicle proliferate rapidly and form the corpus luteum. This ‘yellow body’ produces increasing amounts of progesterone and some oestrogen, and these hormones act on the lining of the womb — it becomes thick and spongy and its glands secrete nutrients that can be used by the embryo if fertilization has occurred: this is the secretory phase of the uterine cycle. The high progesterone level in the blood, together with oestrogen, also exerts negative feedback effects, which decrease the secretion of the gonadotrophin-promoting secretion by which the hypothalamus influences the pituitary. Small amounts of gonadotrophins nevertheless continue to maintain the function of the corpus luteum — but if fertilization does not occur, towards the end of the cycle this support fails and the corpus luteum breaks down. The precise mechanisms which induce this degeneration are unknown, but the consequences are that progesterone and oestrogen secretions decline, the hormonal support of the uterine lining is lost, the spiral arteries contract, and the lining cells, starved of their blood supply, break away. Menstrual bleeding ensues. A new cycle begins.

While the average time for each menstrual cycle is typically depicted as 28 days, cycles do vary considerably in length, ranging from 25 days to 35 days. It is usually the length of the first (follicular) phase of the cycle that accounts for most of the variation. The luteal phase is more likely to last the typical 14 days, with ovulation occurring two weeks before rather than after the onset of menstruation, so it is unpredictable. Furthermore, the luteal phase in some women can also vary. This variability clearly makes ‘safe period’ birth control unreliable.

Saffron Whitehead

Bibliography

Jones, R. E. (1997). Human reproductive biology, (2nd edn). Academic Press, New York.

See also menstruation; premenstrual tension; ova; ovary; sex hormones; uterus.