Encyclopedia of Health and Medicine

244 The Reproductive System
ings) and other genetic information about the
fetus. The amniotic fluid may also provide information
about the woman’s health, such as
whether any INFECTION is present, and help doctors
determine whether the fetus’s lungs are mature.
To perform amniocentesis, the obstetrician first
numbs a small area on the surface of the woman’s
abdomen, either with a topical anesthetic spray or
an injection of local anesthetic. The obstetrician
then inserts a long needle through the woman’s
abdominal wall into the amniotic sac and withdraws
about 20 milliliters (less than an ounce) of
amniotic fluid for laboratory analysis. ULTRASOUND
helps determine the position of the fetus and the
ideal insertion and placement of the needle so as
to avoid injury to the fetus. Because the laboratory
must first cultivate cells from the amniotic
fluid, GENETIC TESTING results take two to three
weeks.
Risks of amniocentesis include bleeding, infection,
injury to the fetus, and spontaneous ABOR-
TION (loss of the pregnancy). Some women feel
temporary discomfort during the procedure, and
many women find the requisite full BLADDER (necessary
for the ultrasound) causes pressure and
other discomforts. Some women experience mild
cramping and slight bleeding for a day or two after
the amniocentesis.
See also ALPHA FETOPROTEIN (AFP); ANESTHESIA;
CHORIONIC VILLI SAMPLING (CVS); CHROMOSOMAL DISOR-
DERS; CONGENITAL ANOMALY; PRENATAL CARE.
amniotic fluid The liquid that surrounds the
developing FETUS within the amniotic sac, a membranous
structure that forms inside the UTERUS in
PREGNANCY. The amnion, the inner membrane of
the amniotic sac, begins producing amniotic fluid
at about two weeks of gestation. The amniotic
fluid, which is mostly water, cushions the fetus
against changes in temperature as well as jarring
and bumps from outside the womb. The composition
of amniotic fluid changes somewhat over the
duration of pregnancy though typically includes,
in addition to water, electrolytes, lipids, proteins,
metabolic byproducts, and cells that the fetus
sheds. These cells provide the DNA that AMNIOCEN-
TESIS uses to assess the health of the fetus.
Amniotic fluid is essential not only to protect
the fetus but also for proper fetal development. In
the second trimester the fetus swallows and
“breathes” to take amniotic fluid into its STOMACH
and LUNGS, which is necessary for development of
the structures and functions of the pulmonary and
gastrointestinal systems. The fetus also begins contributing
URINE to the composition of the amniotic
fluid. By the third trimester the amniotic fluid
replenishes itself about every three hours and
reaches a volume of approximately 500 milliliters.
The amniotic sac ruptures when CHILDBIRTH is
imminent, sending a flood of amniotic fluid from
the woman’s VAGINA. This process is the “breaking
water” that often heralds the onset of pregnancy’s
final stages, labor and delivery.
A lower than normal volume of amniotic fluid
is oligohydramnios, which may constrict the
movement of the fetus to an extent that causes
abnormal musculoskeletal development, intrauterine
growth retardation, and other problems. A
greater than normal volume of amniotic fluid is
polyhydramnios, which may indicate NEURAL TUBE
DEFECTS or BIRTH DEFECTS of the KIDNEYS or gastrointestinal
structures. Polyhydramnios is sometimes
present when the mother has diabetes. It presents
increased risk for UMBILICAL CORD problems such as
umbilical cord prolapse (the umbilical cord enters
the vagina before the fetus’s head as birth begins,
a potentially life-threatening scenario for the
fetus), as well as large for gestational age or
macrosomia (birth weight significantly higher
than normal).
For further discussion of amniotic fluid within
the context of the structures and functions of
reproduction and sexuality, please see the
overview section “The Reproductive System.”
See also CESAREAN SECTION.
andropause A term sometimes used to describe
the physical and emotional changes men experience
at midlife. The amount of TESTOSTERONE, the
primary male sex HORMONE, in a man’s BLOOD circulation
begins to slowly and steadily decline after
reaching its peak in the early to middle 20s. By
age 75, testosterone levels are typically about half
of what they were at age 25. Though this is still an
adequate level of testosterone to maintain masculinity,
the decline accounts for some of the
physical changes characteristic of midlife in men:
conversion of MUSCLE to fat, redistribution of body