This is a series of events that occur every 26-30 days in females throughout the childbearing phase between menarche and menopause. The cycle is made up of a sequence of changes that occur concurrently in the ovaries and uterine lining, which are triggered by variations in hormone concentrations in the blood. Negative feedback systems govern hormones released during the cycle.

The hypothalamus secretes luteinizing hormone releasing hormone (LHRH), which stimulates the anterior pituitary to secrete:

• FSH, which increases ovarian follicle development and oestrogen release, resulting in ovulation. As a result, FSH is mostly active during the first half of the cycle. Its production is reduced after ovulation to prevent further follicles from developing during the current cycle.
• LH, which causes ovulation and encourages the growth of the corpus luteum and progesterone secretion.

The hypothalamus reacts to changes in oestrogen and progesterone levels in the blood. It is triggered by high levels of oestrogen alone (as occurs in the first half of the cycle) but repressed by high levels of oestrogen and progesterone together (as occurs in the second half of the cycle).

The cycle lasts roughly 28 days on average. The days of the cycle are normally counted from the start of the menstrual phase, which lasts about 4 days. This is followed by the proliferative phase (which lasts about 10 days) and the secretory phase (which lasts about 14 days).

Menstrual phase

The corpus luteum begins to deteriorate when the ovum is not fertilised. (During pregnancy, the corpus luteum is aided by human chorionic gonadotrophin, or HCG, which is secreted by the developing embryo.)

As a result, progesterone and oestrogen levels decline, and the functional layer of the endometrium, which is dependent on these ovarian hormones, sheds during menstruation. The menstrual flow is made up of endometrial gland secretions, endometrial cells, blood from degenerating capillaries, and an unfertilized ovum.

Because the corpus luteum, which was active throughout the second part of the previous cycle, has deteriorated, oestrogen and progesterone levels are very low during the menstrual phase.

This means that the hypothalamus and anterior pituitary can continue cyclical activity, and FSH levels begin to rise, kicking off a new cycle.

Proliferative phase

At this stage, one or more ovarian follicles are maturing and generating oestrogen, which drives growth of the functional layer of the endometrium in preparation for the reception of a fertilised ovum.

The endometrium thickens, becoming highly vascular and densely packed with mucus-secreting glands. Rising oestrogen levels are responsible for an increase in LH during the mid-cycle. The LH surge causes ovulation.

Typically, one follicle ruptures and releases its ovum, now known as a secondary oocyte. The proliferative phase has come to an end.

Secretory phase

• Following ovulation, LH from the anterior pituitary drives the growth of the corpus luteum, which generates progesterone, some oestrogen, and inhibin.
• The endometrium becomes oedematous under the effect of progesterone, and the secretory glands generate more watery mucus.
• This facilitates the movement of motile spermatozoa from the uterus to the uterine tubes, where the ovum is normally fertilised.

• The glands of the uterine tubes and the cervical glands that lubricate the vagina secrete similar amounts of watery mucus.
• The fertilisable ovum may survive for a very short time after ovulation, possibly as little as 8 hours.
• Although spermatozoa can live for several days after being implanted in the vagina during intercourse, they may only be capable of fertilising the ovum for about 24 hours.
• This means that the window of opportunity for fertilisation in each cycle is limited.
• Around the time of ovulation, visible changes in a woman’s body occur.
• Cervical mucus, which is ordinarily thick and dry, thins, becomes elastic, and fluid, and body temperature rises by around 1°C immediately after ovulation.
• Some women suffer abdominal discomfort in the middle of their cycle, which is thought to be caused by follicle rupture and release of its contents into the abdominal cavity.
• After ovulation, the corpus luteum’s mix of progesterone, oestrogen, and inhibin suppresses the hypothalamus and anterior pituitary, causing FSH and LH levels to fall. Low FSH levels in the second part of the cycle limit continued follicular growth if the current cycle results in a pregnancy.
• Falling LH levels cause degeneration and death of the corpus luteum, which is dependent on LH for survival if the ovum is not fertilised.
• The resulting progressive reduction in circulating oestrogen, progesterone, and inhibin causes uterine lining degradation and menstruation, as well as the start of a new cycle.
• If the ovum is fertilised, there is no endometrial breakdown and no menstruation. The fertilised ovum (zygote) moves to the uterus via the uterine tube, where it becomes embedded in the wall and produces hCG, which is identical to anterior pituitary luteinizing hormone.

This hormone keeps the corpus luteum intact, allowing it to continue secreting

• progesterone and oestrogen for the first 3-4 months of pregnancy, preventing further ovarian follicle maturation.
• The placenta develops and generates oestrogen, progesterone, and gonadotrophins throughout this time.

Menopause

The menopause (climacteric) often occurs between the ages of 45 and 55, signalling the end of the reproductive era.

It can happen suddenly or gradually over time, sometimes as long as ten years, and is caused by a gradual decrease in oestrogen levels as the number of functioning follicles in the ovaries falls with age.

Ovulation and the menstrual cycle become erratic, eventually slow, as the ovaries become less sensitive to FSH and LH. Other phenomena that may occur simultaneously include:

• Short-term erratic vasodilation with flushing, sweating, and palpitations, producing discomfort and disrupting regular sleep patterns
• Shrinkage of the breasts
• Sparseness of axillary and pubic hair
• Atrophy of the sex organs
• Episodes of unusual behaviour, for eg. irritation, mood changes
• Gradual thinning of the skin
• Loss of bone mass, predisposing to osteoporosis.
• A gradual rise in blood cholesterol levels, which increases the risk of cardiovascular disease in postmenopausal women to the same level as in men of the same age.

Similar changes occur following bilateral irradiation or surgical ovary removal.

Reproductive functions of oestrogen and progesterone

Oestrogen Hormones: Encourages the development of secondary sexual traits during puberty During the proliferative phase, it stimulates and maintains the thickness of the uterine lining. Mid-cycle luteinising hormone (LI) surge, triggering ovulation, In the initial half of the cycle, it stimulates anterior pituitary release of follicle stimulating hormone (FSH) and LH.

Progesterone Hormones: During the secretory phase, it stimulates and supports the thickness and enhanced glandular growth of the uterine lining. In the second part of the cycle, oestrogen inhibits FSH and LH release from the anterior pituitary.