The Menstrual Cycle is a series of events that occurs over a period of time that prepares the lining of the uterus (womb) for pregnancy through the implanting of a fertilised egg. If a pregnancy doesn’t occur, then the lining of the womb that has been built up is shed through menstruation (period).
The cycle varies between 20-30 days in length, but on average it is 28 days (which interesting corresponds with the cycle of the moon!). It is regulated by the production of various hormones (oestrogen and progesterone) by the ovaries which then stimulate the hypothalmus to produce GnRH (gonadotropin releasing hormone), this in turn acts on the pituitary gland which then produces FSH (follicle stimulating hormone) and LH (lutenizing hormone).
FSH stimulates several follicles to grow whilst LH begins the production of oestrogen from the ovaries. At ovulation almost all ovarian follicles die off, leaving just one remaining which continues to develop whilst the uterus is prepared with a thickening layer of endometrium.
At the mid-point of the cycle the pituitary gland is stimulated to produce a large amount of LH, which has remained constant up to this point, this then means that the last follicle matures and releases an egg. It is at this point that progesterone becomes dominant in the cycle.
If fertilisation occurs the egg will be implanted into the uterus and levels of LH and FSH will remain stable. If fertilisation doesn’t occur then the levels of LH and FSH fall. The fall in LH causes a stop in production of hormones and this means that the endometrial lining of the uterus is not maintained and it is then shed as blood.
The fluid is mainly blood, but also includes cervial mucous, bacteria and cells from the endometrium. It is normal to shed around a tablespoon of blood throughout the whole period (although it does look more), but this can vary from woman to woman.