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U N I T 1
Cell and Tissue Function
Prenatal Screening and
Diagnosis
The purpose of prenatal screening and diagnosis is not
just to detect fetal abnormalities. Rather, it has the fol-
lowing objectives: to provide parents with information
needed to make an informed choice about having a child
with an abnormality; to provide reassurance and reduce
anxiety among high-risk groups; and to allow parents at
risk for having a child with a specific defect, who might
otherwise forgo having a child, to begin a pregnancy
with the assurance that knowledge about the presence
or absence of the disorder in the fetus can be confirmed
by testing.
1
Prenatal screening cannot be used to rule out
all possible fetal abnormalities. It is limited to determin-
ing whether the fetus has (or probably has) designated
conditions indicated by late maternal age, family his-
tory, or well-defined risk factors.
Screening and Diagnostic Methods
Amongthemethodsusedforprenatalscreeninganddiagno-
sis are ultrasonography, maternal serum (blood) screening
tests, amniocentesis, chorionic villus sampling, and per-
cutaneous umbilical fetal blood sampling
1,33,60
(Fig. 6-15).
Amniocentesis, chorionic villus sampling, percutaneous
umbilical cord blood sampling, and fetal biopsy are inva-
sive procedures that carry a small risk for the fetus.
Ultrasonography
Ultrasonography is a non-invasive diagnostic method
that uses reflections of high-frequency sound waves to
visualize soft tissue structures. Since its introduction
in 1958, it has been used during pregnancy to deter-
mine the number of fetuses, fetal size and position, the
amount of amniotic fluid, and placental location. It also
is possible to assess fetal movement, breathing move-
ments, and heart pattern. There is also good evidence
that early ultrasonography (i.e., before 14 weeks) accu-
rately determines gestational age.
Improved resolution and real-time units have
enhanced the ability of ultrasound scanners to detect
congenital anomalies. With this more sophisticated
equipment, it is possible to obtain information such
as measurements of hourly urine output in a high-risk
fetus. Ultrasonography makes possible the in-utero
diagnosis of hydrocephalus, spina bifida, facial defects,
congenital heart defects, congenital diaphragmatic her-
nias, disorders of the gastrointestinal tract, and skeletal
anomalies. Cardiovascular abnormalities are the most
commonly missed malformation. A four-chamber view
of the fetal heart improves the detection of cardiac mal-
formations. Intrauterine diagnosis of congenital abnor-
malities permits planning of surgical correction shortly
after birth, preterm delivery for early correction, selec-
tion of cesarean section to reduce fetal injury, and, in
some cases, intrauterine therapy. When a congenital
abnormality is suspected, a diagnosis made using ultra-
sonography usually can be obtained by weeks 16 to 18
of gestation.
Maternal Serum Markers
Maternal blood testing began in the early 1980s with
the test for
α
-fetoprotein (AFP). Since that time, many
serum factors have been studied as screening tests for
fetal anomalies. Current maternal testing uses three
distinct tests (AFP, human chorionic gonadotropin
[hCG], and unconjugated estriol) to screen for trisomy
syndromes in low-risk women while incorporating the
detection of neural tube defects.
61
The combined use
of the three maternal serum markers between 15 and
22 weeks of pregnancy has been shown to detect as
many as 60% of Down syndrome pregnancies.
1
The use
of ultrasound to verify fetal age can reduce the number
of false-positive tests with this screening method.
α
-Fetoprotein is a major fetal plasma protein and
has a structure similar to the albumin that is found
in postnatal life. AFP is made initially by the yolk
sac, gastrointestinal tract, and liver. Fetal plasma
levels peak at approximately 10 to 13 weeks’ gesta-
tion and then decline progressively until term, while
maternal levels peak in the third trimester.
47
Maternal
and amniotic fluid levels of AFP are elevated in preg-
nancies where the fetus has a neural tube defect (i.e.,
anencephaly and open spina bifida) or certain other
malformations such as an anterior abdominal wall
defect in which the fetal integument is not intact.
Screening of maternal blood samples usually is done
between weeks 16 and 18 of gestation.
1,61
Although
neural tube defects have been associated with elevated
levels of AFP, decreased levels have been associated
with Down syndrome.
A complex glycoprotein, hCG is produced exclu-
sively by the outer layer of the trophoblast shortly after
■■
A number of environmental agents can be
damaging to the unborn child, including
radiation, environmental pollutants such
as mercury, medications and illicit drugs,
alcohol, and infectious agents. Because many
of these agents have the potential for causing
fetal abnormalities, often at an early stage of
pregnancy, it is recommended that women of
childbearing age avoid all unnecessary use of
drugs and abstain from alcohol.
■■
The acronymTORCH stands for toxoplasmosis,
other, rubella, cytomegalovirus, and herpes,
which are the infectious agents most frequently
implicated in fetal anomalies.
■■
It also has been shown that maternal folic acid
deficiency can contribute to neural tube defects,
and iodine deficiency can cause congenital
hypothyroidism and impaired neurological
development (cretinism).
SUMMARY CONCEPTS
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