The blood lost in a period results from peeling and shedding of the skin of the inner

lining of the womb. This skin, endometrium, especially develops for each ovulatory cycle so as to provide a fresh bed for implantation of an embryo if an egg is fertilised that month. If fertilisation does not occur, the endometrium is shed so that after the period a new endometrium is formed for the next ovulatory cycle.

Ovulation (egg production) occurs in the ovary under the control of hormones (gonadotrophins), produced by the pituitary gland in the head. These hormones are released into the blood circulation and when they reach the ovary they cause some of the egg follicles to develop.  As the first half of the cycle progresses one of the follicles dominates and grows as a fluid filled cyst up to 2-3cm in diameter.  It secretes another hormone, oestrogen, into the bloodstream.  This oestrogen thickens the endometrial lining of the womb.   Meanwhile, back on the ovary, the follicle ripens and bursts releasing the egg cell, which is picked up by the nearby fallopian tube.  The remaining burst follicle on the ovary switches from oestrogen production to progesterone, a different hormone which matures the endometrium.  The ovary can only keep this up for 2 weeks unless the egg is fertilised. In that case the implanting embryo sends a hormone (hCG) message to the ovary telling it to continue to produce progesterone.  If that message  is not received by 2 weeks after ovulation, progesterone production by the ovary ceases and the endometrial lining cannot be maintained and so starts to peel off.  This lining and the associated bleeding is the is the menstrual flow passed at a period.

This is why ovulation occurs 2 weeks before a period. The hCG mentioned above is the basis for pregnancy tests.  

So a period comes because the progesterone support is lost.  Women on the oral contraceptive pill bleed because the progesterone-like part  of the pill will support the endometrial skin until the pill free week, when the drop in hormones results in the lining being shed.

Irregularity or lack of periods is usually linked in some way to something which has caused a change in the way the body produces progesterone.  Non-ovulation, menopause, pregnancy, thyroid disease is only some of the possible causes of this.  Blood tests can help to investigate some of these causes in women with no abnormalities in the womb. 

During reproductive life the body produces the progesterone in cycle with ovulation and periods will occur if pregnancy does not arise.  Progesterone is necessary to protect the womb.  However the periods themselves are not necessary “to get rid of bad blood”.  It is only natures way of ensuring a fresh bed for the implanting embryo.

Most women have between 11 and 13 menstrual periods each year. You may be different: You may have more or fewer. Missed or irregular periods must be looked at in terms of what is normal for you.

Missed or irregular periods have many causes. Pregnancy is usually the first cause to be considered, but other common causes are:

• Lifestyle changes. Examples include:
• Excessive weight loss or gain.
• Increased exercise (missed periods are common in endurance athletes).
• Extreme emotional stress.
• Travel.
• Medications, such as hormonal birth control methods, which may cause lighter, less frequent, more frequent, or skipped periods.
• Eating disorders, such as anorexia or bulimia.
• Menopause and the 2 to 5 years just prior to menopause (perimenopause).
• The start of menstrual periods (menarche). For the first few years of menstruation, periods may be irregular.
• Hormonal abnormalities, which cause fluctuating levels of the hormones needed to support menstruation.
• Medications other than birth control pills, such as steroids, tranquilizers, diet pills, or recreational drugs.
• Problems with the pelvic organs, such as imperforate hymen or polycystic ovary syndrome.
• Breast-feeding.
• Inflammatory bowel disease.

Investigation after clinical examination includes Ultrasound, Ultrasound scan.

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