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Normal menstrual cycle and periods

During a woman's childbearing years, her body will usually experience a menstrual cycle: a complicated cycle controlled by hormones to prepare her body for pregnancy. This usually lasts around 28 days but varies from woman to woman.

What occurs during a normal menstrual cycle?

When a girl reaches puberty, her reproduction organs - her ovaries (which develop and store eggs), fallopian tubes and uterus (the womb) - have normally matured enough for her to conceive a baby.

Within each cycle, ovulation can occur when an ovary releases an egg into a fallopian tube - a thin tube that runs between the ovary and womb. If the egg is fertilised by a sperm, it can implant into the lining of the womb (the endometrium), which will supply the fertilised egg with the nutrients it needs to develop into an embryo.

If there is no implantation of a fertilised egg, or pregnancy, the womb sheds the nutrient-filled lining of the womb, which is the blood-like flow (or ‘period’) that leaves the womb through the vagina, and the cycle begins again. This cycle is known as the menstrual cycle.

It should be noted that having a menstrual cycle is not a guarantee of ovulation, but indicates that the body's hormones and reproductive organs are working.

How are hormones linked to the menstrual cycle?

Hormones are responsible for regulating the menstrual cycle. The pituitary gland at the base of the brain produces luteinising hormones (LH) and follicle-stimulating hormones (FSH), which in turn stimulate the ovaries to produce two other hormones: oestrogen and progesterone. The latter two hormones trigger the womb and breasts to prepare for pregnancy. There are several phases to the hormone cycle:

  • Follicular phase occurs before the release of the egg
  • Ovulatory phase occurs during the egg release
  • Luteal phase occurs after the egg release.

Follicular phase

The follicular phase starts on the first day of your period:

Two hormones, FSH and LH, are released from the brain and travel in the blood to the ovaries.

The hormones stimulate the growth of about 15 to 20 eggs in the ovaries, each in its own 'shell', called a follicle.

FSH and LH also trigger an increase in the production of the female hormone oestrogen.

As oestrogen levels rise, like a switch it turns off the production of FSH. This careful balance of hormones enables the body to limit the number of follicles that complete maturation, or growth, in any one cycle.

As the follicular phase progresses, one follicle in one ovary becomes dominant and continues to mature. This dominant follicle suppresses all of the other follicles in the group. As a result, they stop growing and die. The dominant follicle continues to produce oestrogen.

Ovulatory phase

The ovulatory phase, or ovulation, starts about 14 days after the follicular phase starts. The ovulatory phase is the midpoint of the menstrual cycle, with the next menstrual period starting about 2 weeks later. During this phase, the following events occur:

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