Your 40-week Journey : Testing, Testing Investigations in pregnancy (part 3) - Diagnostic tests Identifying fetal abnormalities

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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.

Q: How is it done?
A: 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.
Q: What will be checked?
A: 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 .

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.

Q: How is it done?
A: 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.

Chorionic villus sampling test


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.

Q: How is it done?
A: 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.

Amniocentesis test


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…

Q: You shouldn't take baths?
A: 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.
Q: It's not safe to exercise while pregnant?
A: 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.
Q: Pregnant women have that “glow”…?
A: 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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