Cycle

Menstrual Cycle (Hormonal 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 (estrogen 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. 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.

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. — Saffron Whitehead