The cycle

The complex circuit of hormonal stimuli and responses which is responsible for a woman’s monthly periods starts, and ends, at the hypothalamus in the brain the body’s menstrual clock. If the circuit breaks, the monthly period is inevitably disrupted.

Phase one: menstruation

The hypothalamus sends quantities of FSH-releasing factor to stimulate the cells in {he pituitary gland. This is the master gland considered to be the conductor of the orchestra of the body’s other hormone-producing glands, responsible for controlling the ebb and flow of the hormones. The pituitary then begins secreting minute quantities of FSH (follicle stimulating hormone) – and that’s precisely what it does. This hormone stimulates the dormant egg cells which have been lying undisturbed in the ovary since before birth.

Phase two: proliferative phase

The egg cells now begin to develop, acquiring a fluid-filled follicle, like a protective sack, around them. Usually, only one of these follicles will go on to grow enough to force its way through to the surface of the ovary, where it forms a sort of bubble. (Occasionally, two follicles succeed and then twins are a possibility, provided both eggs are fertilized.)

As the follicles are developing in this way, they secrete oestrogen, adding to the quantity already being produced by the ovaries. This rise in the hormone level is a signal to the womb, and stimulates the endometrium (womb lining) to grow.

The lining is made up of narrow tubes, called endometrial glands, set in several layers of cells, called endometrial stromal cells. The oestrogen makes the glands grow, and the layers of the stromal cells increase or ‘proliferate’.

Phase three: ovulation

When oestrogen output is six times higher than its starting level, about 13 days after the onset of menstruation, it has a ‘feedback’ effect on the hypothalamus pituitary, causing the level of FSH-releasing factor from the hypothalamus to drop. Another factor- LH-releasing factor- is sent to the pituitary, stimulating the release of luteinising hormone (LH) which moves into the blood stream with a sudden surge on the 14th day. In combination with the FSH, the hormone induces the most mature follicle bubble in the ovary to burst- releasing the egg.

Phase four: the luteal phase

The newly released egg is caught in the finger like fronds at the end of the Fallopian tube and is wafted slowly, and gently, into the tube itself.

The empty follicle it has left now undergoes its own transformation; it collapses, and, under the influence of LH, its cell walls turn yellow.

It is now known as the corpus luteum – literally the yellow body. The change of colour is, in fact, due to a change of activity. Now, not only does it secrete oestrogen, but also progesterone (or pro-pregnancy), whose main job is to preserve and modify the womb lining. Progesterone thickens the lining and stimulates the glands to secrete a nutritious fluid to nourish any fertilized egg arriving from the Fallopian tubes.

Phase five: pre-menstrual

If no fertilized egg embeds itself, the corpus luteum slowly shrinks and fades away, so that after 12 to 14 days the supply of progesterone is shut off completely, and the level of oestrogen drops back down.

As a result two things happen. First, without the oestrogen and progesterone to maintain it, the thick, juicy endometrium begins to shrink, and in doing so the tiny blood capillaries supplying it become bent- and so break In the deeper layers of the lining, bleeding occurs, separating the lining above the blood, it crumbles and collapses into the womb cavity, along with the blood, eventually causing the womb to contract and expel its debris- a menstrual blood. Second, the restraint on the FSH-releasing factors (which followed the oestrogen surge) is removed , FSH production is stepped up, and the cycle begins all over again.

Sex hormones 1 FSH and LH are both produced by the pituitary gland and are responsible for setting the menstrual cycle in motion. 2 OESTROGEN is produced when FSH stimulates the ovaries. Among its many other functions, oestrogen starts to rebuild the lining of the womb. 3 FSH acts on one of the thousands of immature egg cells in the ovary. The egg matures and comes to the surface of the ovary, where it is contained in a small blister or follicle. 4 About 14 days later LH triggers the release of this mature egg cell, starting it on its journey to the womb. 5 LH stimulates the follicle to burst, releasing the egg from the ovary. This stimulates the production of PROGESTERONE which acts on the lining of the womb, preparing a soft, spongy surface to receive a fertilized egg. 6 If conception does not occur, menstruation begins, clearing out the womb for the cycle to start again.