Antenatal testing describes procedures performed during pregnancy to
detect health problems in the growing fetus; establish characteristics
such as fetal age, sex, or weight; or diagnose any material conditions
that may affect fetal development.
Antenatal tests and exams are important tools for protecting the health of
a pregnant woman and her developing child. Various tests are administered
over the course of pregnancy to determine if the mother has any health
conditions that may interfere with normal development of the fetus or if
the fetus has any health conditions that may affect the baby's
quality of life. Often, families will use information provided by the
tests to prepare for the baby's birth and make arrangements for
special care if needed or make the decision to terminate the pregnancy.
Physicians also use antenatal tests to determine various characteristics
of the fetus, such as gestational age, size, and position in the uterus,
or to verify the presence of multiple fetuses.
Women who become pregnant may undergo tests at any stage in their
pregnancy: during the first trimester (weeks one through 12), second
trimester (weeks 13–26), or third trimester (weeks 27–40+).
What tests are ordered depends on the stage of pregnancy, the age and
health of the mother, the medical history of both parents, and the
family's background or ethnicity. There are two distinct types of
In the early stages of pregnancy, physicians may order blood or urine
tests to screen for possible disorders or infections that could affect the
growing fetus. The tests may also be ordered in later stages if the
pregnant woman comes in contact with an infectious agent or develops
symptoms of infection. In many cases, complications can be avoided if
early diagnosis is made and treatment initiated. Examples of conditions
that are commonly screened for with blood and/or urine tests include:
A multiple marker test or triple screen is used to determine if a fetus is
at an increased risk of having certain congenital abnormalities. The test
has a high rate of false-positives; as few as 10 percent of women with
abnormal results go on to have babies with congenital defects. The purpose
of the test is to determine if further testing (such as ultrasound or
) is warranted. The test requires a sample of maternal blood, typically
taken during the fifteenth and twentieth week of pregnancy, and measures
the level of certain pregnancy hormones.
Ultrasound is a device that records sound waves as they bounce off the
developing fetus and create an image that is projected onto a large
computer screen. A physician may order an ultrasound scan to listen for a
fetal heartbeat, determine a woman's precise due date, or check for
, among other uses. Also known as a sonogram, the procedure takes only a
few minutes, is safe and painless, and usually is covered by health
During the procedure, an ultrasound technician asks the pregnant woman to
remove her clothes and change into a gown. The technician may rub some gel
on the woman's fundus (lower abdomen), which helps the hand-held
device pick up sound waves. In certain cases, the technician may insert a
plastic probe into the woman's vaginal canal to get a clearer
picture of the fetus. Early in pregnancy, the test may need to be done
with a full bladder.
Pregnant women will often have their first ultrasound between eight and 12
weeks of gestation. In normal cases, the technician is able to identify a
fetal heartbeat, which appears as a flashing light on the screen. Closer
to the due date, physicians use ultrasound to make sure the fetus is in
the correct head-first position to exit the birth canal, to assess the
fetus for certain birth defects, and to determine the sex of the fetus if
the parents desire.
Between ten and 14 weeks of gestation, ultrasonography may be used to
measure a small collection of fluid beneath the skin at the back of the
neck. Called nuchal translucency, the measurement tends to be larger in
fetuses with genetic abnormalities such as
, trisomy 13, trisomy 18,
, and triploidy. A particular neck measurement combined with maternal age
as an indicator (e.g. the incidence of the disorder increases in
proportion to the age of the mother) has been shown to correctly diagnose
Down syndrome in 75 to 80 percent of cases; this number increases to 90
percent if the procedure is combined with the multiple marker test.
Amniocentesis is a more invasive test that carries a higher risk of
complications than blood tests or ultrasonography, but is able to
determine more precisely the presence of certain birth defects. It is also
used to determine the level of maturity of the baby's lungs, of
particular interest if the baby will be delivered prematurely. During
amniocentesis, a doctor inserts a thin needle through a woman's
abdomen and into the uterus. Using ultrasound as a guide, the doctor uses
the needle to withdraw a sample of fluid from the amniotic sac. Afterward,
tiny cells shed by the fetus can be studied in the laboratory; scientists
can analyze the samples to determine if the fetus has certain genetic
conditions. Amniocentesis is typically performed during the second
trimester of pregnancy and particularly in mothers over the age of 40.
Chorionic villus sampling (CVS) is a procedure that allows for prenatal
diagnosis during the first trimester (generally between ten and 12 weeks
of gestation, during the embryonic stage of development). The test
taking a small sample of cells from the placenta with a needle through
the abdominal wall or a small tube (catheter) through the cervix. The
procedure is guided by ultrasound. The cells are then analyzed in a
laboratory for the presence of genetic abnormalities.
The risks associated with antenatal testing depend on the specific
procedure being performed:
Alpha fetoprotein test
—A screening blood test that can be done after the sixteenth week
of pregnancy to evaluate the possibility of spina bifida and other
neural tube defects in the fetus.
—A procedure performed at 16–18 weeks of pregnancy in
which a needle is inserted through a woman's abdomen into her
uterus to draw out a small sample of the amniotic fluid from around the
baby for analysis. Either the fluid itself or cells from the fluid can
be used for a variety of tests to obtain information about genetic
disorders and other medical conditions in the fetus.
—The foot-down or bottom-down position of a fetus just before
Group B streptococcus
—A serotype of streptococcus, Streptococcus agalactiae, which is
beta hemolytic and can cause neonatal sepsis, pneumonia, or meningitis
if present in the birth canal at the time of delivery especially when
the delivery is difficult.
—An iron-containing pigment of red blood cells composed of four
amino acid chains (alpha, beta, gamma, delta) that delivers oxygen from
the lungs to the cells of the body and carries carbon dioxide from the
cells to the lungs.
—An infection or inflammation of the membranes that cover the
brain and spinal cord. It is usually caused by bacteria or a virus.
—A medical test in which sound waves are directed against
internal structures in the body. As sound waves bounce off the internal
structure, they create an image on a video screen. Ultrasonography is
often used to diagnose fetal abnormalities, gallstones, heart defects,
and tumors. Also called ultrasound imaging.
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