Encyclopedia of Health and Medicine

304 The Reproductive System
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 (ovulation).
Without pregnancy the blood levels of estrogens
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 pregnancy
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 thyroid
gland) is a common source of such hormonal
disruption. Disorders of the pituitary gland, such
as pituitary ADENOMA, or the ADRENAL GLANDS, such
as ADRENAL INSUFFICIENCY, often alter the body’s
endocrine matrix in ways that affect the menstrual
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 OVARIES ends the menstrual
cycle. OOPHORECTOMY is the surgical OPERATION to
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 also DYSFUNCTIONAL 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 BLOOD and excess tissue that build up within the
UTERUS as 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 napkins
(commonly called pads) or tampons to capture
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 overflow
and maintain appropriate PERSONAL HYGIENE.
Tampons may irritate the vaginal walls. Because a
tampon may carry BACTERIA into the vagina when
the woman inserts, tampon use involves a slight
risk for TOXIC SHOCK SYNDROME, a potentially lifethreatening
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 GENITALIA with
gentle soap and warm water is important to
cleanse any accumulated menstrual fluids from
the genital tissues, which reduces the presence of