Whether prenatal 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.
Here’s a low-down on the screening tests available for the prenatal detection of Down's syndrome:
1. 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.
Description: 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.
A mother looks forward to the ultrasound examination because it gives her a first glimpse of her baby. At the same time, doctors and midwives use the ultrasound to gather a wealth of information about the baby and the pregnancy.
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.
2. Nuchal Scan
When is it done: 11-14 weeks.
Detection rate: 75% for Down's syndrome.
Description: 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.
3. 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%.
Description: 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.
4. CVS: Chorionic Villus Sampling
When is it done: Usually between 9-14 weeks.
Accuracy: Almost 100%.
Description: 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.
Description: 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.