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Menstuation (also known as the menstrual cycle or the period ) refers to the monthly haemorrhage caused by the the shedding of the lining of the uterus (termed the endometrium ). Althought this cycle occurs in roughly monthly cycles, the actual number of days in any individual woman's cycle can be anywhere between 20 and 40 days.

Menstruation reflects a woman's reproductive years and extends from the menarche (first menstruation) Menstruation begins in puberty and ends with menopause . By definition, the first day of bleeding is counted as the beginning of each menstrual cycle (day 1). The cycle ends just before the next menstrual period. Menstrual cycles range from about 21 to 40 days. Only 10 to 15% of cycles are exactly 28 days.
The intervals between periods are generally longest in the years immediately after menarche and before menopause.

The menstrual cycle is typically separated into three phases, follicular , ovulatory , and luteal , which begin with the first day of menses (bleeding).

  1. The follicular phase , is the phase which begins with the first day of menses (Day 1). During the early part of this phase, the pituitary gland increases release of Follicle Stimulating Hormone (FSH) and Luteinising Hormone (LH). FSH stimulates the growth of numerous follicles in the ovary as well as the release of oestrogen from the ovary. The oestrogen causes thickening of the endometrium. However, only one of the follicles matures and gets to release the ovum (egg) contained within. LH is secreted in a sudden burst mid-cycle (circa Day 14) and stimulates the release of the ovum from the mature follicle in the ovary.

  2. The ovulatory phase is where the ovum is released from the mature follicle in a process termed ovulation, due to the increased presence of the hormone LH. Ovulation generally occurs in the 16 to 32 hours following follicle exposure to increased LH levels. The follicle ruptures and releases the ovum into the funnel-shaped end of the fallopian tube . The ovum then makes its way down the tube and into the uterus, hoping to get fertilised on the way.

  3. The luteal phase is the period following ovulation, which continues until the next menstrual period. Following ovulation the follicle that ruptured, releasing the ovum, closes and becomes a corpus luteum . The corpus luteum produces increasing levels of oestrogen as well as progesterone . Both hormones have the effect of preparing the endometrium for the implantation of a fertilised egg. If pregnancy does not occur, the corpus luteum regresses and the levels of progesterone and oestrogen decline. This results in the breakdown of the endometrium and menstrual bleeding occurs. If pregnancy does occur, the corpus luteum begins to produce human chorionic gonadotropin (HCG). It is the top and middle endometrium layers that are lost during the menstrual period. The bottom layer remains and prepares once again for implantation in the next cycle.

Menstrual function may be disturbed in a number of ways. The main types of menstrual dysfunction can be seen here .

The information in this page is presented in summarised form and has been taken from the following source(s):
1. Introduction to Human Physiology , 2nd Edition (1981), M. Griffins, Macmillan Publishing Co. Inc, New York
2. Child Development , 6th Edition (1994), J. W. Santrock, Wm. C. Brown Communications, Inc.

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