The normal menstrual cycle

Menstrual periods are part of the female reproductive cycle. They start during puberty (between the ages of 10 and 16) and continue until the menopause (45 to 55). The average menstrual cycle lasts 28 days, but can vary between 24 and 35 days. Menstrual bleeding normally lasts between two and seven days, with the average being five days.

The pattern of a woman's menstrual cycle can be disturbed by:

  • changing the contraception method
  • an imbalance of the hormones, which regulate the menstrual cycle.

Causes of irregular periods


The following factors can upset your balance of hormones and cause irregular bleeding:

  • extreme weight loss or gain,
  • heavy exercise
  • stress.


It is not unusual to have a hormone imbalance for a few years after puberty and before the menopause, which can cause irregular bleeding. The menstrual cycle may become longer or shorter and/or the periods may become lighter or heavier.

Gynaecological problems

Irregular bleeding can also be due to more serious problem, such as:

  • cervical or endometrial polyps - benign growths in the lining of the cervix or womb cavity,
  • uterine fibroids
  • ovarian cysts
  • pelvic inflammatory disease (PID) - infection in the pelvis which can cause pelvic pain and bleeding after sexual intercourse or between periods,
  • cancer of the womb (although this is very rare).

Polycystic ovarian syndrome

Polycystic ovarian syndrome (PCOS) occurs when many cysts (small, fluid-filled sacs) develop in the ovaries. The usual symptoms of PCOS are irregular or light periods, or no periods at all.

This is because in PCOS, ovulation (the releasing of an egg) often does not take place. Also, the production of hormones may be unbalanced with higher levels of testosterone (a male hormone that women have a small amount of) than normal.


An intrauterine device (IUD) or contraceptive pill may cause spotting between periods. An IUD can cause heavy periods.

Small bleeds, known as breakthrough bleeds, are common when the contraceptive pill is first used. They are normally lighter and shorter than normal periods, and usually stop within the first few months.


Bleeding during sexual intercourse or heavy bleeding between periods could be a symptom of cancer of the cervix or womb, although this is rare. This can be diagnosed by examination and biopsy. Tests may include colposcopy, direct examination of the vagina and cervix and a pelvic scan.

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