Written by: Dr Patrick Chia, Consultant Obstetrician and Gynaecologist, Fetal Medicine & Gynaecology Centre
Antenatal tests are now routinely employed to detect abnormalities in the baby. Advances in fetal imaging such as ultrasound, genetics and minimally invasive techniques and the knowledge of the natural history of fetal diseases, have changed the way doctors manage such pregnancies. This information affords the doctor the opportunity to offer antenatal procedures such as laser therapy, fetal surgery or other fetal intervention necessary. It will also enable the doctor to tailor or manage themode and timing of delivery in order to optimize the outcome for both mother and baby.Furthermore, it will help the neonatal specialists prepare for a potentially ill baby at delivery.
1. What are fetal abnormalities?
Fetal abnormalities describe a group of disorders found in the baby from birth. Also known as congenital disorders or birth defects, they include congenital malformations, deformations and chromosomal abnormalities. The structure and function of an organ(s) is(are) often affected.
WHO estimates that some 260 000 deaths worldwide (about 7% of all neonatal deaths) were caused by fetal abnormalities in 2004. Recent studies have shown that they are responsible for the major health problems in newborns. More than a quarter of all deaths in the first year of life are attributed to fetal abnormalities.
2. Why do we test for fetal abnormalities?
To allow in-utero treatment when appropriate or delivery at a tertiary centre for immediate postnatal therapy.
Allow termination of a seriously affected baby especially if the mother’s health is affected as well
To prepare the parents psychologically, socially, financially, and medically for a baby with a health problem or disability or for the likelihood of a stillbirth.
Having this information in advance of the birth means that healthcare staff as well as parents can better prepare themselves for the delivery of a child with a health problem. For example, Down syndrome is associated withcardiac defects that may need intervention immediately upon birth.
3. What tests are available?
Antenatal testsinclude both screening and diagnostic tests (Figure 1). The screening tests, being non-invasive, do not jeopardize or pose any risk to the pregnancy. They do not cause a miscarriage. However, their sensitivity is not 100%, which means that the detection rate is not 100%. In this context, screening tests for Down syndrome do not completely detect or would it exclude the condition. Examples of screening tests for Down syndrome include ultrasound scan and the triple test.
The invasive tests, however, are diagnostic tests which include amniocentesis, chorionic villous sampling and fetal blood sampling. They are called diagnostic tests since they will tell for certain if the baby has the condition—in this case, Down syndrome. The tests involve sampling the baby’s cells for analysis. Since they are invasive tests, there is a small risk of miscarriage (between 0.5 – 2%).
Figure 1:Antenatal tests to detect abnormalities in the baby involve the non-invasive and invasive tests. The non-invasive tests are screening tests and do not carry a risk of miscarriage. However, they do not give a definite answer. The invasive (or diagnostic) tests, on the other hand, give a definitive result but carry a small risk of miscarriage. Screening tests do not give a definite answer, but they do tell us which babies have a higher risk (increased risk). Diagnostic tests are then offered to the women at higher risk.
Antenatal screening will provide estimates of the chances or risks that particular pregnancies are affected by disorders such as Down syndrome and help parents in making choices about diagnostic procedures that carry risks for their unborn babies. The process would therefore frequently involve the difficult reconciliation of uncertainties about their pregnancy outcomes in the context of personal moral values and religious sentiments.
4. When are the tests performed?
Figure 2: Timeline of antenatal tests to detect abnormalities in the baby
Early ultrasound scan utilizes the high resolution trans-vaginal probe to detect fetal abnormalities before 10 weeks gestation. Coupled with 3D technology, exquisite details of the embryo can be seen and abnormalities such as spina bifida have been detected as early as 8 weeks gestation.
The first trimester screening (FTS) test for Down syndrome combines both an ultrasound scan and a blood test is now available. This is done at 11 to 14 weeks gestation, normally offered at 12 weeks. It has to be done during this “window” period when the baby (CRL or crown-to-rump) measures between 45 and 84 mm. The nuchal scan as it is called, measures the nuchal translucency (or NT; a translucent area beneath the skin at the nape of the neck) and the presence of the nasal bone (Figure 3). This is combined with a blood test, which measures the levels of two placental hormones called PAPP-A (pregnancy associated plasma protein A) and the free β-HCG (beta human chorionic gonadotrophin). The values of these four parameters are then entered into a computer software, which then calculates a personalized risk of Down syndrome. The FTS is a good screening test since it can detect up to 95% of babies with Down syndrome.
During the nuchal scan, efforts are made to ascertain normal anatomy of the rapidly developing fetus. Major structural anomalies can be detected at this time. Other early markers of chromosomal disorders may be apparent during this scan. They include pyelectasis (swollen kidneys) and hyper-echogenic bowel (bowel looks bright on ultrasound scan).
Figure 3: Measuring the nuchal translucency, NT at 12 weeks. A diagnostic test is offered if the NT measurement is >2.5mm
If the FTS’s “window period” is missed, the second trimester screening (STS) test for Down syndrome can be done from 15 weeks until about 22 weeks gestation. The triple test, a blood test, measures the levels of three pregnancy hormones which are alpha feto-protein (αFP), unconjugated oestriol (uE3) and free β-human chorionic gonadotropin (β-HCG). The “double test” measures two of these hormones. Levels of αFP and uE3 typically rise during the second trimester, while β-HCG decreases. The quadruple test (QT) involves the measurement of an additional substance, Inhibin-A, in the maternal blood. It increases the detection rate of Down syndrome to about 80%.The triple test, however, detects about 60-70 % of Down syndrome.
The detailed ultrasound scan or fetal anomaly scan is offered between 18 to 22 weeks gestation. Usually done at 20 weeks, it ensures that the baby does not have any physical or anatomical defects such as spina bifida, cleft lip and palate and congenital heart defects. Apart from being able to detect structural abnormalities, it will also look for markers of chromosomal disorders such as Down syndrome (Trisomy 21). Major ultrasound markers include congenital heart defects (e.g. ventricular septal defect or VSD) and duodenal atresia (blocked duodenum). This “genetic scan” will also look for minor or soft markers such as nuchal thickening, renal pelvis dilatation and hyper-echogenic bowel.
5. How do I interpret the result?
The results are normally in the form “one in …..,” for example , “one in 100” or “one in 1000”. The numbers indicate the probability or likelihood of your baby having Down syndrome. The result “one in 1000” means that there is one chance in 1000 that your baby has the condition. As the second number gets bigger, the chance of your baby having Down syndrome gets smaller.
If the screening test is lower than the acceptable “cut-off” risk of 1 in 250, then you will be offered a diagnostic test. This “cut-off” risk is rather arbitrary but it would seem more sensible and logical to compare the calculated risk with your background risk (or your age related risk). For example, if a woman is aged 40, her age related is one in 100 or 1% and if her calculated risk (based on the screening test) is one in 200, it would be reassuring that her risk is now reduced by half and yet she is still screened “positive” if we use the “one in 250 cut-off “ level. A diagnostic test would be offered to her despite her “lower” risk.
6. What happens if I am screen positive?
About one in 30 screening tests will be “positive” which means that the risk is higher than the “cut-off” risk of one in 250. This does not mean that your baby has Down syndrome. You will be offered a diagnostic test such as an amniocentesis to confirm or exclude the condition.
7. What if I am screened negative?
You would be screened negative if the risk is lower than the cut-off risk of one in 250, say, one in 500. However, having a negative screen does not guarantee that the baby does not have Down syndrome. It just means that the risk is lower.
8. Is there a role for 3D and 4D ultrasound in the detection of fetal abnormalities?
Rapid advances in imaging techniques, particularly in 3D 4D ultrasonography has catapulted us from an era of 2D sonography to theera of 3D“volume” ultrasound. This new technology has given us an armamentarium of new capabilities such as multi-planar analysis, tomographic views, surface rendering, volume and fetal weight estimation, 3D angiography, spatio-temporal image correlation (STIC), volume contrast imaging (VCI) to view the fetus.
3D ultrasound has emerged as an invaluable tool in for the detection of fetal abnormalities. It had been shown that 3D ultrasound is superior to 2D in evaluating certain abnormalities such as those involving the face (Figure 4), thorax, limbs, spine and central nervous system. Consulting specialists understand fetal abnormalities better and can therefore optimize the counseling process and plan prenatal or postnatal intervention.
3D 4D ultrasound has undoubtedly become a powerful tool in the visualization of the baby and pertinent in the area of fetal abnormalities detection, it has afforded doctors the diagnostic confidence and accuracy, hence changing clinical practice to optimize care for the mother and her baby.
Figure 4: 3-D ultrasonography can display details of the normal face (a) and confirm abnormalities such as cleft lip (b).
9. What diagnostic tests are available?
Diagnostic tests available include amniocentesis, chorionic villous sampling (CVS) and fetal blood sampling (FBS). Diagnostic tests are able to confirm for sure if the baby has the condition being tested. Where a genetic disorder is suspected, such as Thalassaemia (an inherited blood disorder), a gene probe for the defect is used to test for the condition. Since these tests are “invasive” (involves sampling with a needle or small cannula/tubing) there is a procedure-related risk of miscarriage between 0.5 to 1%.
Amniocentesis is the commonest of the specialized tests and involves drawing some amniotic fluid that surrounds the baby, for analysis. Under ultrasound guidance a sample of amniotic fluid is aspirated using a spinal needle inserted through the abdominal wall (Figure 5). It is performed from 15 weeks onwards, usually at 16 weeks of pregnancy.
Figure 5: Amniocentesis in progress
CVS is essentially a placental biopsy, and is done earlier, usually 10-12 weeks of pregnancy. Placental tissue contains cells that are genetically similar to the baby’s. Women with increased risk of chromosomal abnormalities (those with a positive FTS or increased NT measurement) or genetic diseases (such as α and β-thalassaemia) may be offered a CVS.
Depending on where the placenta is situated, CVS can either be done trans-abdominally or trans-cervically (through the neck of the womb). The procedure is similar to an amniocentesis but a local anaesthetic is used. Under ultrasound guidance, a sample of placental tissue (chorionic villous) is obtained using a needle that is inserted through the abdominal wall (Figure 6).Very occasionally, the placenta cannot be accessed abdominally. The trans-cervical approach using a fine cannula (tubing) has to be performed.
Fetal blood sampling (FBS) is a highly specialized test, which involves taking a small amount of blood from the umbilical cord of the baby for analysis. It is usually done when a structural (physical) abnormality is found on ultrasound scan. This test is usually performed after 20 weeks gestation and has the highest risk of miscarriage, between 2-3 %. However, it is a rapid way of analyzing the baby’s blood to confirm the suspicion of a major abnormality such as chromosomal disorder or thalassaemia major. FBS is seldom done now since most conditions can be diagnosed from fetal cells extracted from amniotic fluid and CVS.
Figure 6: A trans-abdominal CVS in progress.
It is normal for a mother to be worried if the baby they are carrying is normal. Many tests are now available to test for abnormalities. In most cases screening tests are used to reassure the parents. Diagnostic tests, on the other hand, will confirm if there is an abnormality. Once confirmed, timely medical or surgical treatment can be instituted before or after birth. It will also prepare the parents psychologically, socially and financially.
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