http://en.wikipedia.org/wiki/Menstrual_cycle
The menstrual cycle is the scientific term for
the physiological changes that occur
in fertile women and other female primates for the purposes of sexual reproduction. This article
focuses on the human menstrual cycle, a "monthly" cycle that takes
approximately 28 days to complete although this does vary from
person to person.
The menstrual cycle, under the control of the endocrine system, is necessary for
reproduction. It is commonly divided into
three phases: the follicular phase, ovulation, and the luteal phase. It is also occasionally
misclassified using the uterine cycle: menstruation, proliferative phase, and secretory
phase.[1] Menstrual cycles
are counted from the first day of menstrual bleeding. Hormonal
contraception interferes with the normal hormonal changes with
the aim of preventing reproduction.[2]
Stimulated by gradually increasing amounts of estrogen in the follicular phase, discharges of
blood (menses) slow then stop, and the lining of the uterus thickens. Follicles in the ovary begin developing under the influence of a complex
interplay of hormones, and after several days one or occasionally
two become dominant (non-dominant follicles atrophy and die).
Approximately mid-cycle, 24–36 hours after the Luteinizing Hormone
(LH) surges, the dominant follicle releases an ovum, or egg in an event called ovulation. After
ovulation, the egg only lives for 24 hours or less without
fertilization while the remains of the dominant follicle in the
ovary become a corpus luteum; this body
has a primary function of producing large amounts of progesterone. Under the influence of
progesterone, the endometrium (uterine lining) changes to prepare
for potential implantation of
an embryo to establish a pregnancy. If implantation does not occur within
approximately two weeks, the corpus luteum will involute, causing
sharp drops in levels of both progesterone and estrogen. These
hormone drops cause the uterus to shed its lining and egg in a
process termed menstruation.
In the menstrual cycle, changes occur in the female reproductive
system as well as other systems (which lead to breast tenderness or mood changes, for example). A woman's
first menstruation is termed menarche, and occurs typically around age 12-13. The
average age of menarche is about 12.5 years in the United States,[3] 12.72 in
Canada,[4] 12.9 in the
UK[5] and 13.06 ± 0.10
years in Iceland.[6] The end of a
woman's reproductive phase is called the menopause, which commonly occurs somewhere between
the ages of 45 and 55.
u want to help... just help lah... if you don't want to help,
then don't help... why you so funny go and paste the whole
dictionary here? we know what is PMS lah... we not stupid.