A cycle in female humans and some higher primates of reproductive age, during
which the body is prepared for pregnancy.
In women, the average cycle is 28 days. At the beginning of the cycle, hormones
from the pituitary gland stimulate
the growth of an ovum (egg cell) contained
in a follicle in one of the two ovaries.
At approximately mid-cycle, the follicle bursts, the egg is released (ovulation)
and travels down the fallopian tube
to the uterus. The follicle (now called the
corpus luteum) secretes two hormones,
progesterone and estrogen,
during this secretory phase of the cycle, and the endometrium
thickens, ready to receive the fertilized egg. Should fertilization
(conception) not occur, the corpus luteum degenerates, hormone secretion
ceases, the endometrium breaks down, and menstruation occurs; the unfertilized
egg is discharged in the blood flow from the vagina.
|The changes occurring during the menstrual cycle
are controlled by the balance of the follicle stimulating hormone
(FSH) and luteinizing hormone (LH) secreted by the pituitary. The
diagram shows the changing levels of these, and of estrogen and progesterone
induction from the ovarian follicle, together with changes in the
structure of the uterine wall (A) and development of the follicle
(B), in a circular form through a normal 28-day cycle. The sharp increase
in LH at about mid-cycle causes ovulation (C) and, fertilization does
not occur, the corpus luteum (D) formed degenerates around day 26
as pituitary hormone levels fall. The consequent withdrawal of estrogen
and progesterone causes the uterine wall to shed itself in the menstrual
flow. This then proliferates again under the influence of estrogen
from a new follicle. If fertilization and egg implantation do occur
the placenta produces chorionic gonadotrophin, possibly as early as
day 21, that allows the corpus luteum to continue to produce estrogen
and progesterone until the placenta takes over.
In the event of conception, the corpus luteum remains and maintains the
endometrium with hormones until the placenta
is formed. Menstruation, therefore, marks the end of the cycle. It is customary,
for the purposes of contraception
and calculating the date of an expected birth, to count the first day of
bleeding as the first day of the next menstrual cycle. The onset of the
menstrual cycle (menarche) is called puberty
(age 10–15 years); it ceases with the menopause (around 50 years).