Prenatal diagnostics encompass all prenatal examinations designed to detect hereditary diseases or physical malformations in the unborn child. These procedures are of particular relevance to women with certain risk factors
Testing for genetic or chromosomal disorders A screening test is conducted in mothers who may be at risk. Based on the results of this test, a decision will be made as to whether a more invasive test should be carried out.
Today, most chromosome disorders can best be discerned by means of the so-called First Trimester Test (the optimum period for this test is between the 11th and the 14th week of pregnancy). This test takes into account the mother's age, the results of a maternal blood analysis and the thickness of the unborn child's neck fold, measured by ultrasound.
Ultrasound scan Ultrasound examinations are conducted before pregnancy, to check fertility or cyclic changes in the uterus and ovaries and once pregnancy is established, to check on the health of the pregnancy and the growing embryo.
During the scan the operator will put lubricating gel on your abdomen to help the probe used to make good contact with your skin. The probe is connected by a wire to the ultrasound machine and monitor. Pulses of ultrasound (high definition sound waves) are sent from the probe through the skin into your body. The ultrasound waves then echo or bounce back from the various structures in the body.
The echoes are detected by the probe and sent down the wire to the ultrasound machine. They are displayed as a picture on the monitor, which is constantly updated, allowing the scan to show movement as well as structure.
The doctor uses ultrasound to measure embryo precisely, in order to determine the length of the pregnancy and the date of delivery. The scanner also makes it possible to recognise malformations early on, by measuring the neck fold.
The first ultrasound test takes place between the 10th and 14th week of pregnancy and the second between the 20th and the 23rd week. There are no known risks for the child in the ultrasound testing.
The doctor can usually recognise arms and legs, as well as the heartbeat and any movement of the embryo, early on in the pregnancy.
The best time to examine the child's organs is in the 20th - 23rd week, at which point, the organs are large enough to permit a detailed sonographic examination and it is possible to clearly see the baby's face.
Considerations in the first trimester:
- The heart is visibly beating
- Position of the foetus in the uterus
- Stage of pregnancy reached
- Multiple or single pregnancy
- Possible chromosome disorders (e.g. Downs Syndrome)
Considerations in the second and third trimester:
- Quantity of amniotic fluid
- Extent of foetal growth
- Evidence of defects
- Position of the placenta
- Doppler examination in the case of high risk pregnancies (blood circulation in the placenta and foetus)
- 3D ultrasound
In South Africa, only two ultrasound examinations are covered by health insurance. A medical indication would be needed to justify further scans.
Measuring the neck fold
Between the 10th and 14th week of pregnancy, the skin fold in the neck of the child is measured, using ultrasound imaging. The thicker the fold, the larger the risk for Downs Syndrome. Using this simple method enables 80% of those affected to be identified.
A thickened neck fold can also be due to harmless causes. This is why the testing of the neck fold may be combined with an examination of the mother's blood (serum screening).
Maternal serum screening (blood test)
The blood test can be done between 14 weeks and 20 weeks plus 6 days to screen for neural tube defects such as spina bifida (abnormal development of part of the spine and spinal cord) and anencephaly (severely abnormal development of the brain). If you are having the second trimester blood test for Downs Syndrome, neural tube defect screening can be done at the same time. If the test shows that your baby has an increased chance of having a neural tube defect an ultrasound will be done straight away. A detailed ultrasound scan of the baby when you are around 18 - 20 weeks pregnant can detect almost all babies with a neural tube defect (95%).
If the results of your screening tests indicate the possibility of a malformation, a more precise testing method can be applied such as Chorionic Villus Sampling (CVS) or Amniocentesis.