Menarche signifies beginning of Menstruation in a female. Age at Menarche is always asked or whenever a woman goes for Gynecological or obstetrical treatment.
Understanding Menarche would entail understanding MENSTRUATION
WHAT IS MENSTRUATION?
Menstruation is the cyclical shedding of the inner lining of the uterus that manifests as periodic bleeding from the vagina occurring throughout a womans reproductive life. First menstruation for a girl (menarche) signals the onset of her reproductive life that usually ends with menopause (stoppage of the cycles)
During the first years of menstruation may be irregular, but most women eventually establish a regular pattern. It is common for intervals to be as brief as 21 days or as long as 45. Some women never become entirely regular, but this does not affect their health. Irregularity is also common in the older woman just before menopause.
A woman of any age needs a physical examination when her irregularity is more than slight, as when there are many months between periods.
WHAT HAPPENS DURING MENSTRUATION?
The uterine wall is made up of two layers, first a muscular layer myometrium (comprises almost 99% of the uterine wall) and the endometrium - inner thin layer of cells, to which the foetus gets attached during pregnancy. Accordingly, every month the endometrium has to be prepared to accept the embryo. This happens through a series of hormonal fluctuations.
In order to understand what exactly happens, lets understand the hormonal changes.
The hypothalamus, which is situated in the brain, controls all the hormones of the body. In this case hypothalamus releases a hormone that called as gonadotropin-releasing hormone in turn acts on the pituitary gland which in turn releases hormones acting on the Thyroid gland. Hormones produced by the thyroid gland ultimately act on the ovary to produce oestrogen and progesterone - the female sex hormones.
Without going into intricacies let us remember that there is more of oestrogen in the first half of the cycle whereas there is more of progesterone in the latter half.