Diagnosis of genetic disorders can be done before birth, but after the embryo had implanted in the uterus. This is called Pre-Natal Diagnosis, and is carried out in antenatal (before birth) clinics, attended by pregnant women.
Several techniques are in common use:
- maternal blood sampling
- ultrasound scanning
- chorionic villus sampling
Maternal blood sampling
This is a routine procedure of antenatal clinics, but neural tube disorders of the foetus can be indicated at an early stage by a raised level of alpha-fetoprotein, a specific protein in the woman's blood,.
This is a very widely used technique, and is routinely done in antenatal clinics for early confirmation of foetal heartbeat and to estimate foetal size as the pregnancy advances. It is also very useful for the detection and diagnosis of gross abnormalities. Ultrasound technology has advanced considerably in recent years, and it is now possible to make detailed measurements of various foetal body functions such as breathing movements, and the speed of the foetal blood flow.
In this test a hollow needle is passed through the walls of the woman’s abdomen and uterus, and a sample of amniotic fluid is withdrawn. This contains cells from the foetus, which after culture can be tested for chromosomal and genetic abnormalities. It cannot be done before 16 weeks of gestation timed from the date of the last menstrual period (14 weeks from fertilisation), and results are not usually available before the 20th week of pregnancy. If the outcome of the test indicates a severe abnormality, the woman may be offered a termination of pregnancy at this stage. There is a slight risk of the procedure of amniocentesis itself bringing about a miscarriage.
Chorionic villus sampling
The advantage of this test over amniocentesis is that it can be carried out earlier in the pregnancy. A needle is inserted via the cervix and guided by ultrasound scanning towards the placenta. Here, a sample of the placental tissue is collected. Similar tests upon these cells may then be carried out, and because there are usually more in the sample than the number obtained from amniocentesis, results are generally more quickly available, because the cells do not have to be cultured prior to testing.
The foetus can be directly visualised by insertion of a fibre optic foetoscope. This may be done in addition to amniocentesis, or instead of it, in which case samples of tissue may be taken directly from the foetus or umbilical cord. As with amniocentesis, there is a small but significant risk of spontaneous abortion.
Tests are now available for about 60 genetically inherited diseases and syndromes. Some of these are very rare, and are only offered to families known to be at risk.
See pages on Designer Babies
Move on to pages on Genetic Discrimination