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PRENATAL SCREENING

1. The Value of Prenatal Screening

Most babies born are healthy, and there is often no need for concern. However, most parents want some reassurance that their unborn baby is not suffering from various possible birth defects, such as physical deformities, chromosomal abnormalities and genetic diseases. Many defects can be discovered during pregnancy by one or more prenatal tests, but how much you want to know is up to you.

Some women feel that routine testing is unnecessary for them since the risk of serious defects is small. And, they are also prepared to deal with any minor defects after the baby is born. This is a perfectly acceptable pregnancy plan.

Other women feel that whatever the outcome, pregnancy is a gift, and they would manage any eventuality when it arises. They too may not be keen on any form of prenatal testing. This too would be considered a parent’s right to decide.

Many parents feel better ruling out common defects, especially those for which their baby is at higher risk. Many couples opt for prenatal testing so that they can be better prepared for the birth of a special needs child. Prepared couples tend to cope better than those who discover only at birth that their child would require special care in the early years of life.

Support groups such as the Down Syndrome Association, Singapore have special arrangements to help couples with babies in whom prenatal tests have detected fetal abnormalities. Also, babies with certain defects may need to be delivered under special conditions such as by caesarean section. Or, they may require special resuscitation equipment and special teams to be on hand at the time of birth. If the birth defect is a very serious or potentially fatal one, knowing in advance may also help parents prepare emotionally for the situation their child faces. Having some time to know what to expect — and to accept — can allow the family of a child with a fatal defect to make the most of all the time they have available with their unborn child.

It is important to remember that this unfortunate circumstance is very uncommon.

In most cases, prenatal testing is reassuring, and it allows parents to continue to enjoy their pregnancy and bond with their unborn baby. Bonding is known to start as early as the second trimester of pregnancy.


2. Dating Scan

One of the most important things to know about your baby is her ‘dates’. This means how old your baby is. Actually, by convention we measure your fetus’ age from the first day of your last normal menstrual period. It’s an important estimate on which many decisions about your baby will depend, such as when to perform the nuchal translucency scan, when to do the amniocentesis, and when to deliver the baby.

The dating scan accurately determines the fetal age, but at this scan important additional information can be gathered such as the viability of the fetus, the number of fetuses, and in the case of twins whether they’re fraternal or identical twins. A slow fetal heart rate in pregnancy may indicate a higher risk of miscarriage, but a good, fast fetal heart rate in early pregnancy suggests a very good chance that the pregnancy will continue nicely.


3. Screening and Diagnostic Tests

Prenatal testing includes both screening tests and diagnostic tests. Screening tests are non-invasive and usually involve a simple blood test and an ultrasound scan. Screening tests however do not always identify the abnormality, but would be able to assess if there is a low or high risk of an abnormality. Diagnostic tests on the other hand can determine for certain if a fetal abnormality is present (e.g. Down syndrome), but it usually involves an invasive test such as amniocentesis and is associated with a risk of fetal miscarriage.
Research has led to new advances in Non Invasive Prenatal Screening. Non-invasive for prenatal care means no deployment of any invasive means near the fetus, more specifically, no needling near the fetus. It is anticipated that these new tests will very significantly reduce the number of invasive diagnostic tests such as Amniocentesis and Chrorionic Villus Sampling (CVS) by as much as 95%. These invasive tests carry a risk of miscarriage of about 1 in 100.


4. Nuchal Translucency (NT) Scan


The Nuchal translucency scan is usually done between the 11th and 14th week of pregnancy. This test can estimate the likelihood of a baby having Down syndrome (Trisomy 21) and identifies women who should be offered further testing.

The nuchal translucency is a pocket of fluid at the back of the baby’s neck, which can best be seen between the 11th and 14th weeks of pregnancy. Babies with Down syndrome tend to have a larger collection of fluid than healthy babies. Using ultrasound, the amount of fluid is measured and this measurement is fed into a computer, along with the mother’s age to compute the risk of the baby having Down syndrome at the time of the clinic visit. The risk of the mother delivering a baby with Down syndrome is usually half that because some babies with Down syndrome would miscarry naturally before reaching term.


5. Combined Test: Nuchal Translucency (NT) Scan + First trimester serum screen

In this test, the results of the NT Scan are combined with the blood results of two pregnancy specific hormones: b-human chorionic gonadotrophin (b-hCG) and pregnancy-associated plasma protein A (PAPP-A).This test can detect 84% of pregnancies where the fetus has Down syndrome, and is currently the standard test for screening Down syndrome. Newer non invasive screening tests are now becoming available with the detection rate of 99%. (hyperlink to NIPT).

Other tests are being developed to enhance the non-invasive detection of Down syndrome such as nasal bone evaluation, blood flow in specific fetal vessels (ductus venosus) and more serum markers, but these are not yet in routine clinical practice.


6. Fetal DNA in Mother’s Blood

In the pregnant mother’s blood circulation are free DNA molecules – and about five per cent of that free DNA is derived from fetal cells. This is called Cell-free DNA (cfDNA) and given that they are actual fetal fragments of DNA circulating in the pregnant mother’s blood, they are considered to be a more direct and accurate form of obtaining information on the fetus, as opposed to visual screening methods such as ultrasound, which is reliant on the skill of the operator.

The Non Invasive Prenatal Test (NIPT) are able to detect chromosomal abnormalities in the fetal cfDNA fragments with 99.96% accuracy, rendering it a near diagnostic. 10ml of blood is required for the test to ensure there is enough cell free fetal DNA for analysis.


7. Triple Test & Quadruple Test


Triple tests and quadruple tests are blood tests that measure similar biochemical substances in mother’s blood: alphafetoprotein (AFP), human chorionic gonadotropin, unconjugated estriol, and inhibin A. The first three are measured in both tests; inhibin A is included in the quadruple test. Both tests are usually performed between the 15th and 22nd week of pregnancy. Of course, the earlier the better.

High levels of AFP could indicate that the fetus has a neural tube defect such as spina bifida or anencephaly. Low AFP levels, on the other hand, and abnormal levels of human chorionic gonadotrophin could indicate that the fetus has chromosomal abnormalities such as Down syndrome (Trisomy 21) or Edward syndrome (Trisomy 18).

The triple and quadruple tests are screening tests. Like all screening tests, these tests can only indicate that the fetus may have a problem. Further testing, by invasive prenatal methods are necessary to confirm any abnormality.


8. Amniocentesis


Amniocentesis is a prenatal diagnostic test that’s carried out between the 15th and 20th weeks of pregnancy.

The amniocentesis procedure involves a fine needle being inserted into the fetal amniotic sac to obtain up to 20 ml of amniotic fluid for analysis. The amniotic fluid is analysed for fetal cells and fetal DNA to study chromosomal and genetic disorders such as Down syndrome, thalassemia and cystic fibrosis. This is a highly versatile test that can also detect many other genetic disorders. This test is usually recommended for pregnant women aged over 35 because of the increased risk of her baby carrying a chromosomal abnormality. These days however, even mothers over the age of 35 can choose to do a screening test first, and to do an amniocentesis only if they find the find the calculated risk unacceptable.

Using standard, but older, technology, amniocentesis results take between 8 to 21 days to be ready. Different labs differ in their reporting times. Modern molecular technologies such as AmnioPCR (or QFPCR) now allow some results to be available within 48 hours (unless it’s over a weekend), and even newer techniques have been developed that allow laboratory testing to be completed within two hours. FlashFISH™ technology allows, for the first time ever, Same Day Prenatal Diagnosis.
Amniocentesis is associated with a small, but not insignificant, risk of fetal miscarriage of about 1%. For now at least the Royal College of Obstetricians & Gynaecologists quotes this figure in their Patient Information Leaflet, although this figure is expected to decline as more evidence is gathered in the literature.


9. Chorionic Villus Sampling


Chorionic villus sampling (CVS) is another prenatal diagnostic test that is usually carried out between the 11th and 13th weeks of pregnancy. At CVS, a fine needle is inserted into the placenta and a tiny piece of the placenta (chorionic villus) is obtained for analysis.

Just as with the amniocentesis, CVS is carried out to obtain fetal cells and fetal DNA to study chromosomal and genetic disorders such as Down syndrome, thalassemia and cystic fibrosis. Similarly as well, it also is a highly versatile test that can detect many other genetic disorders.

This test is usually recommended for pregnant women known to be at increased risk for chromosomal and genetic disorders such as Down syndrome and thalassaemia. CVS is associated with a slightly higher risk of fetal miscarriage of about 1.3% – 2%


10. Cordocentesis


This diagnostic test, which examines blood from the fetus to detect fetal abnormalities is also known as fetal blood sampling and percutaneous umbilical cord blood sampling (PUBS).

Using ultrasound to determine the location where the umbilical cord meets the placenta, or the fetal liver, a thin needle is inserted into the umbilical cord to collect a small sample of fetal blood.
This procedure, which is similar to amniocentesis except that fetal blood is retrieved instead of amniotic fluid, is generally performed after the 20th week of pregnancy.

Like amniocentesis, cordocentesis tests for chromosomal abnormalities and blood disorders. Cordocentesis can also help diagnose fetal anaemia and Rh isoimmunisation. The risk of miscarriage related to cordocentesis is about 2%.