The 18–22 week screening scan Your baby's physical examination
Also known as the
fetal screening scan, this detailed scan is offered to all women between
the 18th and 22nd week of pregnancy. At this stage of gestation, your
baby has well-developed limbs and facial features and all its major
organs and body systems are in place and can be assessed.
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How is it done?
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The scan involves transmitting high-frequency sound waves
through the uterus that bounce off the baby, and the returning sounds
are converted into an image (see Ultrasound scans).
The biggest echoes are from hard tissues, such as bones, which appear
white in the image on the screen, while soft tissues are gray-flecked.
Fluid-filled spaces, such as the stomach, bladder, blood vessels, and
amniotic fluid surrounding the baby, do not return sound waves so appear
black. It is the difference between echoes and colors that enables the
ultrasonographer to interpret images.
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What will be checked?
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The ultrasonographer starts by checking the fetal heartbeat and
then counts the babies—rarely, twins are not revealed until 20 weeks!
She will measure the head circumference and diameter (biparietal
diameter), and the abdominal circumference and the femur (thigh bone) to
date the pregnancy and ensure your baby is growing well. She will check
for abnormalities in the brain, face and lips, spine, abdomen, heart,
stomach, kidneys, bladder, and hands and feet. Lastly, the placenta,
umbilical cord, and amniotic fluid are examined. You may be able to find
out the sex of your baby, although you can ask not to be given this information
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Diagnostic tests Identifying fetal abnormalities
Diagnostic tests
give a definitive answer as to whether or not your baby has an
abnormality such as Down syndrome. These tests are not performed
routinely and you will be offered one only if a screening test indicated
that your baby had a higher risk for Down syndrome, if you are 35 or
over, or you have a family history that puts you at a higher risk of
having a baby with an abnormality. All diagnostic tests also carry a
small risk of miscarriage and you will need to weigh up the pros and
cons of the tests before deciding to go ahead with one.
Chorionic villus sampling (CVS)
This is a diagnostic test
that involves taking a sample of tissue from the placenta to identify
for certain whether your baby has Down syndrome or a genetic
abnormality. This can be determined since the placenta contains the same
genetic information as the baby. The test is carried out between 10 and
13 weeks of pregnancy. The advantage of this test is that it can be
performed earlier in pregnancy than amniocentesis, so if an abnormality
is found and you decide to terminate, it is early enough to have a
suction termination.
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How is it done?
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There are two procedures for CVS; one method extracts a sample
of the placenta via the abdomen, and the other method carries out the
procedure vaginally. With the abdominal method, a fine needle is
inserted through your abdomen and, using an ultrasound scan for
guidance, the doctor removes a very small sample of tissue from the
placenta. You have to wait about three days for the results, which means
that if your baby has an abnormality and you want to terminate your
pregnancy, you can do so well before you start to feel your baby
kicking.
To perform CVS
vaginally, the doctor inserts a small tube through your vagina and the
cervix, which then passes through the uterine wall. As with the
abdominal method, the doctor then takes a small sample of tissue from
the placenta, using ultrasound for guidance. The sample is sent to a
laboratory, where it is grown in a culture for around seven days. The
sample is then studied under a microscope to check for chromosomal
abnormalities or other defects.
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Chorionic villus sampling test
Amniocentesis
Amniocentesis is a
diagnostic test used mainly to identify a chromosomal abnormality and it
is the most commonly used test for identifying Down syndrome in a baby.
During the test, a sample of amniotic fluid containing cells from the
baby's system is taken from the uterus. It is a relatively quick and
painless procedure and may be offered at around 15–19 weeks of
pregnancy. Amniocentesis is offered later than CVS (see image)
because there may be insufficient fetal cells in the amniotic fluid
before this stage of pregnancy. The results from this procedure are
usually very accurate and, although there is a slight risk of
miscarriage, this is lower than the risk of miscarriage with CVS,
especially in centers where a large number of the tests are routinely
performed and the doctors are particularly practiced at conducting the
test. Apart from the slight risk of miscarriage, the main disadvantage
of amniocentesis is that it has to be performed later in pregnancy, so
if the result comes back as positive, then you will be half way or even
further into your pregnancy should you decide to terminate and would
need to be induced to undergo a vaginal delivery.
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How is it done?
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Using an ultrasound scan to guide the procedure, a long, thin
needle is inserted through the mother's abdomen into the amniotic sac
and a small sample of amniotic fluid is extracted. This contains fetal
cells, which are then grown in a culture in a laboratory to be analyzed.
Strenuous exercise and intercourse should be avoided for 48 hours
following the procedure. Notify your doctor if you develop fever,
bleeding, cramping or leaking of fluids. Depending on the laboratory,
results are typically available within 7–10 days.
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Amniocentesis test
Cordocentesis
This is a diagnostic test
that is also known as “blood sampling” or “percutaneous umbilical
sampling” (PUBS). During this procedure, a sample of blood is taken from
the baby's umbilical cord and this is used to diagnose chromosomal
defects when earlier screening tests have indicated a possible problem.
However, since this is an extremely specialized procedure, it is
performed by a perinatologist at a speciality center.
Myths and misconceptions Is it true that…
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You shouldn't take baths?
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This isn't
true. Excessive heat (above 101° F/38° C) isn't good for babies, but
taking a warm bath or shower shouldn't increase your core body
temperature too much. Just make sure the
water isn't too hot, and avoid hot tubs and saunas. Bathing shouldn't
cause a vaginal infection either, although if your water has broken you
shouldn't sit in standing water without consulting your midwife.
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It's not safe to exercise while pregnant?
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Quite the opposite—gentle exercise throughout your pregnancy will boost your energy, keep you mobile, and relieve stress.
You shouldn't start any new, vigorous activities, and avoid high-impact
exercise, but walking, swimming, or prenatal yoga are ideal. Also, if
you're taking an exercise class or going to the gym, make sure your
instructor knows you're pregnant.
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Pregnant women have that “glow”…?
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Many people believe that pregnancy causes a woman's skin to glow.
All the hormones produced by your body at this time may have beauty
benefits, such as thicker hair and faster-growing nails. But while some
may bask in a rosy glow from the increased blood volume churning through
their bodies, others endure broken blood vessels and spider veins.
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