Estrogen is also essential for maintaining the
content of calcium and other bone-building min-
erals. After menopause calcium more easily leaves
the bones and is less easily absorbed into the blood
circulation from dietary sources, a double effect
that can rapidly result in osteoporosis. More than
two thirds of women over age 65 have some
degree of osteoporosis. Calcium supplementation
in combination with RESISTANCE EXERCISE (also
called weight-bearing exercise) helps the bones to
retain the calcium they require to remain dense
and strong.
See also AMENORRHEA; BONE; BONE DENSITY; CHO-
LESTEROL, ENDOGENOUS; EXERCISE AND HEALTH; GENERAL
ANXIETY DISORDER (GAD); HYSTERECTOMY; MEDICINAL
HERBS AND BOTANICALS; MENARCHE; PREMATURE OVAR-
IAN FAILURE(POF).
menstrual cramps See DYSMENORRHEA.
menstrual cycle The pattern of hormonal and
physiologic changes that occur that occur in a
woman’s body in preparation for possible PREG-
NANCY. Though the average menstrual cycle spans
28 days, the frequency of MENSTRUATION varies
widely among women and often within each
woman individually. Menstrual cycles may be as
short as 25 days or as long as 32 days and still be
within the range of normal. Menstrual cycles out-
side these parameters may or may not be normal,
depending on the woman’s individual physiology
and health status. The endocrine system directs
the menstrual cycle.
Physiologic Phases of the Menstrual Cycle
There are four phases within the menstrual cycle
that always occur in the same order:
- The proliferative phase begins with the end of
menstruation and the return of the
endometrium (lining of the UTERUS) to its non-
menstrual state and culminates with OVULATION
about 14 days after the onset of menstruation.
During proliferation the level of ESTROGENSin
the BLOOD circulation rises and the level of
PROGESTERONE drops. The changing hormone
levels stimulate the maturation of up to 20 ova
within their ovarian follicles, called ripening.
The follicle containing the first ovum to reach
full maturity ruptures and releases the ovum
into the fluid surrounding the fimbriae (fluted
edges) of the fallopian tube. The other follicles
that had started to develop then shrink; the
ovary absorbs them and their ova.- The expelled ovum leaves behind the corpus
luteum, a structure of endocrine tissue that
begins secreting estrogens and progesterone.
This period of activity by the corpus luteum is
the luteal phase, also called the secretory phase.
The increased blood levels of the hormones
cause the endometrium to thicken and its blood
vessels to enlarge. The glands that line the
endometrium increase their secretions, and the
inner endometrium becomes spongy and
engorged in preparation to support implanta-
tion should CONCEPTIONoccur. The luteal phase
lasts about 10 days. - When the ovum passes through the uterus
without implanting, the corpus luteum invo-
lutes (turns in on itself) and the follicle absorbs
it. The sudden drop in estrogens and proges-
terone causes the endometrial blood vessels to
contract, called endometrial ischemia. The
endometrial glands stop their secretions and
the endometrium dramatically shrinks. This
third phase of the menstrual cycle, called the
ischemic phase, lasts 36 to 48 hours. - The culminating phase of the menstrual cycle is
menstruation, during which the anemic (blood-
deprived) tissue of the endometrium sloughs
away and passes from the body. The menstrual
flow contains tissue fragments, endometrial
secretions, and blood. Menstruation lasts 3 to 5
days in 85 percent of women; about 15 percent
of women menstruate for 7 days. Though men-
struation is the last phase of the menstrual
cycle, doctors consider the first day of men-
strual bleeding to be the start of the menstrual
cycle.
Endocrine Regulation of the Menstrual Cycle
The HYPOTHALAMUS, PITUITARY GLAND, and corpus
luteum regulate the menstrual cycle. The hypo-
thalamus initiates the proliferative phase of the
menstrual cycle by releasing GONADOTROPIN-RELEAS-
ING HORMONE(GNRH). GnRH stimulates the pituitary
gland to secrete a surge of FOLLICLE-STIMULATINGmenstrual cycle 303