Menstrual cycle

Menstrual cycle

The normal menstrual cycle consists of two phases where different hormones act in specific ways to prepare for pregnancy:

First phase

• The pituitary - a small, pea-sized gland located at the base of the brain - releases a hormone (follicle-stimulating hormone or FSH) which helps follicles in the ovary (structures holding eggs) to mature.
• At the same time, the ovaries start to produce more oestrogen to thicken the inner lining (endometrium) of the womb.
• An ovum (egg cell) capable of being fertilised develops within the ovary each month. About halfway through the cycle the ovum becomes mature.
• Rising levels of oestrogen in the blood then trigger another pituitary hormone (luteinising hormone or LH) that causes the ovum to be released from the ovary and into the fallopian tubes (see diagram) through which it travels to the womb. If the egg is fertilised by a sperm, pregnancy starts.

A woman’s reproductive organs

Second phase

• After the ovarian follicle ruptures and releases the egg, a new structure, the corpus luteum (Latin meaning: yellow body) develops and remains in the ovary.
• The corpus luteum produces the hormone progesterone in high quantities.
• Progesterone helps to prepare the womb to receive a fertilised egg, maintains pregnancy, and helps develop the breasts in readiness for breast-feeding.
• If the egg does not become fertilised the corpus luteum shrinks, production of progesterone stops, the endometrium disintegrates, and menstruation begins.
• The resulting low levels of oestrogen and progesterone stimulate the pituitary to release the follicle-stimulating hormone so that the menstrual cycle starts again.

OK

Menstruation occurs during a woman's reproductive years as part of the normal menstrual cycle where hormones prepare the lining of the womb (endometrium) to nourish a fertilised egg should it implant.

In fertile women the ovaries release a single mature egg each month. If the egg remains unfertilised as it passes through a woman's womb, a hormonal feed- back system ensures the lining of the womb is shed soon after and this is experienced as a period.

Menstruation lasts between 2-7 days losing approximately 35ml to 50ml of blood (7 to 10 teaspoons). After menstruation the lining of the womb is restored as the body is once more under the influence of oestrogen.

Usually by the time a woman is in her mid 40s, ovulation occurs less frequently. Many women experience much heavier periods at some times and no period at all at others until eventually menstrual bleeding stops altogether.

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