- Antenatal tests can be done to detect genetic or chromosomal abnormalities.
- There are two types of tests: screening tests and diagnostic tests.
- Diagnostic tests are ultrasound scan, nuchal scan, AFP test, maternal serum triple test, Chorionic Villus sampling and amniocentesis.
Antenatal testing has become available at several centres. However, it is not required simply because a medical practitioner offers it to expectant parents and it is not everybody’s choice to have such tests performed.
It is important to realise that any antenatal test can lead to a confrontation with an unexpected abnormal result. This may lead to difficult decisions regarding the option of pregnancy termination, a decision some parents may feel they do not want to face. Thus, for many women, the decision to have the tests is a difficult one because of the serious implications of the information provided.
Many families are not always sure that having the tests is the right decision for them as they may be bound by moral, ethical or religious issues. For this reason no antenatal tests should be performed without in-depth discussion with, and informed consent from the parents.
In general, women with a family history of genetic abnormalities are encouraged to obtain genetic counselling about potential antenatal tests available. Women over 35 years of age may be offered antenatal testing as the risk of a baby having Down’s syndrome increases with maternal age.
Whether antenatal tests are being done during pregnancy for reassurance of the parents or to prepare them or to offer choices in case an abnormality is detected, it is always useful to know what the various tests offer and what their limitations are before giving consent.
Antenatal tests can be divided into two categories:
- Screening tests
- Diagnostic tests
Screening tests are usually done by taking a blood sample from the mother or by performing an ultrasound scan. The results can be given in two forms.
- A pregnant woman is given a “one in x” chance of having a baby with a particular condition. This figure means exactly what is says. If, for example, a certain feature is found in a baby which has a risk of one in 500 that the baby will be born with a particular medical condition, then this means that in a group of 500 babies with the same characteristics one will be affected and 499 will be normal. This is obviously a low risk but it is important to understand that the screening test cannot distinguish between the one affected baby and the other 499 unaffected ones. A risk of 1 in 5 still leaves you with a 80% (4 in 5) chance that the baby is not affected. Although this is certainly regarded as a high risk of abnormality (20%), the chances that the baby is normal are still four times greater.
- Another way of interpreting a test is in the form of a "positive" or "negative" result. This just means that doctors and laboratories have set a fixed number (e.g. 1 in 250) as the cut-off point for the risk of a certain condition. Any risk lower than this threshold is called "negative". Any risk higher than this is "positive". These terms may be confusing since a negative result does not mean for certain that a particular baby is not affected - it only means that the risk is lower than 1 in 250. On the other hand, a positive result does not indicate automatically that a baby must be abnormal - in fact the vast majority of babies in that risk group are absolutely normal.
There are different types of screening tests for the detection of a medical condition. These tests may, if compared with each other, have different detection rates regarding the number of affected babies they detect in a group of women who have an abnormal result.
Most private obstetricians offer some of these tests routinely to all pregnant women, while others only offer testing to older women.
Before embarking on the various tests, parents must realise that they have a 1 in 20 chance of being confronted with a worrying result and that they then have to face the decision regarding invasive diagnostic testing.
Diagnostic tests give a conclusive answer about the presence of chromosomal abnormalities. However, they are invasive procedures that involve taking fluid or tissue from around the baby by introducing a needle into the uterus (womb). They can take weeks before giving a definitive answer, and all have in common a slight risk of leading to a miscarriage.
Ultrasound scan (Sonar)
When is it done: Any time during pregnancy. When screening specifically for chromosomal abnormalities, the best time for the scan is between 11 and 14 weeks. For structural defects, better views are obtained later on in the pregnancy.
Accuracy: This depends on the experience of the sonologist and on the condition being looked for.
Ultrasound imaging has been increasingly used in obstetrical care since the 1970’s.
The ultrasound uses high frequency sound waves which are bounced off the baby. A computer then translates the echoing sounds into video images which reveal the baby's shape, position and movements.
Mothers look forward to the ultrasound examination because it gives them a first glimpse of their baby. At the same time, doctors and midwives use the ultrasound to gather a wealth of information about the baby and the pregnancy.
The following are some of the uses of the ultrasound:
- Assessing the viability of a pregnancy - The ultrasound has the ability to quickly and accurately establish whether a foetus is dead or alive and to predict whether a pregnancy is likely to continue after a threatened miscarriage. It allows early identification of a failed pregnancy before the patient experiences any signs that indicate a miscarriage.
- Assessing the number of foetuses and the type of twins - With ultrasound, multiple pregnancies can be detected at a stage where they are not clinically suspected. This may improve the management of such pregnancies. Ultrasound is the only way to determine whether the twins are identical or not. This is very important since both types of twins require a different approach in the care and management.
- Dating a pregnancy, checking the heartbeat and measuring growth - Using an ultrasound, the gestational sac can be seen from five to six weeks and the foetus by week six or seven. As soon as the foetus can be visualised, it can be measured and its viability confirmed by the detection of heart movements. The use of ultrasound to assess the duration of the gestation is based on the fact that in early pregnancy, foetal growth is rapid and there is little biological variation in size. In the first trimester, the measurement of the foetal crown–rump length is usually accurate within seven days. After 13 weeks, this measurement becomes less reliable and other methods (measuring the head, the tummy and the thigh bone) are used. Measurement of the foetus after 30 weeks is of no value for assessing the duration of pregnancy since at this advanced stage, there is too much variation between big and small babies of the same gestational age.
- Checking for foetal structural abnormalities directly. A large number of foetal abnormalities can now be detected by modern ultrasound imaging. A good sonologist can detect abnormalities such as a cleft lip and palate, spina bifida, major limb or brain abnormalities and any severe abnormalities of the heart or kidneys. The presence of one defect may suggest the presence of others, which are more difficult to detect with ultrasound. Another test such as an amniocentesis may be used if the findings suggest the presence of a chromosomal abnormality. The early detection of severe abnormalities in the developing foetus offers parents the option to consider termination of the pregnancy. Identification of a treatable condition may lead to intervention that can save the baby's life. Identification of other types of abnormalities may have the advantage that the paediatrician is forewarned and prepared to treat a baby that may be unwell at birth.
- Locating the placenta - The ultrasound is the best available method for locating placental position and to rule out placenta praevia (= obstructing the birth canal). It is also important prior to chorionic villus sampling and amniocentesis.
Some hospitals are equipped with 3D ultrasound machines. With the three-dimensional scan, a computer rapidly stores a series of images to create a life-like picture of the baby and doctors are able to see a cross-section of any part of the body. The 3D scan is usually considered an extravagance rather than a necessity and its use in the detection of abnormalities is limited.
The main purpose of scanning in late pregnancy is to identify a foetus with intrauterine growth restriction which may benefit from elective delivery.
When is it done: 11-14 weeks.
Detection rate: 75% for Down's syndrome.
The mother has an ultrasound scan whereby the collection of fluid at the back of the baby’s neck is measured. A high reading may point to abnormalities such as Down’s syndrome or a heart defect. If this is the case, another diagnostic test such as amniocentesis or CVS (Chorionic Villus Sampling) is needed to provide a more conclusive answer regarding the chromosomes. Scans later in pregnancy need to be done to rule out cardiac problems.
The nuchal scan is the single most effective screening method for Down's syndrome provided the operator is properly trained and accredited by the Foetal Medicine Foundation (London, UK). Names of their accredited centres in South Africa can be found on their web site.
When is it done: 16-18 weeks.
The alpha-fetoprotein (AFP) blood test is a test that screens for possible abnormalities of a baby. A baby's liver secretes a sugar protein called alpha-fetoprotein and this protein will be evident in the mother's blood. Elevated levels of AFP may indicate defects such as spina bifida (open spine) or anencephaly (absence of the brain and part of the skull) or open abdominal (tummy) wall defects.
Low levels suggest Down's syndrome or other chromosomal defects. Should the test results not read at normal levels, it could lead to additional tests such as an amniocentesis.
The AFP test does not determine the existence of disorders; it only predicts their increased likelihood. The AFP test is often done in conjunction with an ultrasound and can be done either alone or in addition to other blood tests. The results are usually returned within a few days.
Maternal Serum Triple Test (also known as the Down’s test)
When is it done: 15-22 weeks.
Detection rate for Down's syndrome: 65%.
This is a blood test that is done to measure the levels of three hormones. The test can be an indicator of a baby’s risk of neural tube defects (spina bifida) or Down’s syndrome. Care must be taken to make sure that the dating of the pregnancy is correct. Inaccurate dating may cause these hormone levels to be incorrectly interpreted. If the test is positive, it is not a positive indicator that a baby has Down’s syndrome, but rather that the mother is at a higher risk for a Down’s baby at her age compared to the norm. An invasive test like amniocentesis is still required to establish whether the baby is affected or not.
An extension of this test is the Quadruple test which tests for four chemical markers and is slightly more accurate.
The screening tests on maternal blood can be combined with the nuchal fold test to give a more accurate prediction of Down's syndrome.
CVS: Chorionic Villus Sampling
When is it done: Usually between 9-14 weeks.
Accuracy: Almost 100%.
This test is usually offered to women who are considered high risk for having babies with genetic disorders such as cystic fibrosis or chromosomal abnormalities.
A sample of the cells that line the placenta, known as chorionic villi, are extracted either by using a fine needle which passes through the abdomen (transabdominal CVS) or via a catheter which is inserted through the cervix (transcervical CVS) into the placenta. The sample is then analysed. Because no amniotic fluid is retrieved during the procedure, CVS cannot detect neural tube disorders.
The results take about one to three weeks depending on the type of condition being detected. Although there is a risk of miscarriage when doing this test, it is estimated at only around 1%, provided the operator is as experienced in this technique as in amniocentesis.
When is it done: Any time from 14 weeks, but ideally between 16 – 18 weeks.
Accuracy: Virtually 100% for chromosomal abnormalities.
A long needle is inserted through the mother´s abdomen and her uterus until it has reached the amniotic fluid around the baby. An ultrasound scan guides the needle to avoid any contact with the baby or placenta. A small amount of fluid is aspirated and sent to the laboratory for analysis. The baby’s cells in the withdrawn amniotic fluid need two to three weeks to grow before the chromosomes in a single cell can be examined for any chromosomal disorders such as Down’s syndrome.
Other tests can detect for example open spinal defects. Performing an amniocentesis has a 0.5-1% risk of causing a miscarriage.
Reviewed by Prof Bert Schaetzing, MD, FCOG(SA), FRCOG, PhD, part-time consultant, Dept of Obstetrics & Gynaecology, Faculty of Health Sciences, University of Stellenbosch