Facts on File Encyclopedia of Health and Medicine

HORMONE(FSH), which induces an ovarian follicle
to begin secreting estrogens. The estrogens cause
the ovum within the follicle to begin ripening. The
rising level of estrogens in the blood circulation
triggers the hypothalamus to again release GnRH,
which this time stimulates the pituitary gland to
secrete LUTEINIZING HORMONE (LH). LH causes the
ovarian follicle to produce progesterone, which
brings the ovum to maturity and release (ovula-
tion). Without pregnancy the blood levels of estro-
gens and progesterone both fall and menstruation
takes place.

Disturbances of the Menstrual Cycle

Numerous factors may disrupt the menstrual
cycle, the most common being pregnancy. When a
fertilized ovum (ZYGOTE) implants in the
endometrium, the menstrual cycle ends and preg-
nancy begins. The menstrual cycle does not return
until six to eight weeks (and sometimes longer, up
to months in women who are breastfeeding) after
CHILDBIRTH. Hormonal imbalances may also disrupt
the menstrual cycle. HYPOTHYROIDISM(underactive
THYROID GLAND) or HYPERTHYROIDISM(overactive thy-
roid gland) is a common source of such hormonal
disruption. Disorders of the pituitary gland, such
as pituitaryADENOMA, or the ADRENAL GLANDS, such
as ADRENAL INSUFFICIENCY, often alter the body’s
endocrine matrix in ways that affect the men-
strual cycle.
Numerous medications and treatments such as
CHEMOTHERAPY and RADIATION THERAPY may affect
ovarian function. Menstrual disturbances may
occur as a result of underlying health conditions
such as POLYCYSTIC OVARY SYNDROME(PCOS), OBESITY,
anorexia, and extreme emotional or physical
stress. Though a normal menstrual cycle often
occurs with a single functioning ovary, the
absence or loss of both OVARIESends the menstrual
cycle. OOPHORECTOMYis the surgical OPERATIONto
remove an ovary. Women who participate in
intense athletic activities, such as marathons and
triathlons, may have irregular menstrual cycles or
AMENORRHEA(absence of menstruation).
For further discussion of the menstrual cycle
within the context of the structures and functions
of reproduction and sexuality, please see the
overview section “The Reproductive System.” For
further discussion of the menstrual cycle within

the context of the structures and functions of the

endocrine system, please see the overview section

“The Endocrine System.”

See alsoDYSFUNCTIONAL UTERINE BLEEDING(DUB);

DYSMENORRHEA; FALLOPIAN TUBES; FERTILITY; INFERTIL-

ITY; MENARCHE; MENOPAUSE; PREMATURE OVARIAN FAIL-

URE(POF); PREMENSTRUAL SYNDROME(PMS).

menstruation The final phase of the MENSTRUAL

CYCLE, commonly called the menstrual period or

simply the period. Menstruation is the discharge

of BLOODand excess tissue that build up within the

UTERUSas endometrium (the lining of the uterus)

thickens and engorges with blood in preparation

for the implantation of a fertilized ovum (ZYGOTE).

When PREGNANCY does not occur, hormonal

changes cause the lining to slough away, passing

from the uterus and out of the body via the

VAGINA. Typically a woman passes two to three

ounces of blood and other fluids over the course

of the three to seven days she menstruates. The

menstrual flow is generally heaviest on the second

through the fourth days. About 85 percent of

women menstruate for four to five days; about 15

percent menstruate for six to seven days.

Women typically use disposable sanitary nap-

kins (commonly called pads) or tampons to cap-

ture the menstrual flow. Pads have adhesive strips

that attach them to underwear; tampons fit inside

the vagina. It is important to change either pads or

tampons every four to six hours to prevent over-

flow and maintain appropriate PERSONAL HYGIENE.

Tampons may irritate the vaginal walls. Because a

tampon may carry BACTERIAinto the vagina when

the woman inserts, tampon use involves a slight

risk for TOXIC SHOCK SYNDROME, a potentially life-

threatening INFECTION. Doctors recommend using

pads at night and during other times when it

might not be possible or practical for a woman to

change her sanitary protection every four to six

hours.

There are no health reasons for women to

avoid their regular activities, including sports,

bathing, and sexual activity if desired, during

menstruation. Women may prefer to shower

when menstruating. Washing the GENITALIAwith

gentle soap and warm water is important to

cleanse any accumulated menstrual fluids from

the genital tissues, which reduces the presence of

304 The Reproductive System