Tests to detect genetic abnormalities.
These may be advised if there is any family history of genetic disease or if the mother is over 35 years of age. The most common genetic disorder is Down’s syndrome. About half the children with Down’s syndrome are born to mothers over 35 years of age. Other genetic disorders that can be diagnosed before 20 weeks include cystic fibrosis, hereditary disorders of red blood cell formation such as thalassaemia, and many rare disorders of body chemistry resulting from congenital lack of particular enzymes. Hereditary disorders that are sex-linked (transmitted through the mother’s genes to all or half of all conceptions, but causing disease only in male offspring) can be predicted by detecting whether the foetus is male.
The chromosomes in the nuclei of foetal cells must be examined to detect genetic abnormalities. Chromosome tests will also discover the sex of your foetus, which you can be told if you wish. Foetal cells may be obtained in several ways.
This is a test to analyse the foetus’s chromosomes and genes. A sample of amniotic fluid can be obtained by inserting a hollow needle through the abdominal wall and the wall of the uterus into the amniotic sac. The procedure is done with local anaesthetic and the guidance of ultrasound. Amniocentesis takes about 10 minutes, though the needle is in the amniotic sac only for about 30 seconds. The miscarriage rate after amniocentesis is one in a hundred or less.
The fluid obtained is centrifuged to separate the cells from the liquid. The cells are placed in a nutrient broth and incubated until they have grown enough to be examined. This can take from a few days up to four weeks. When the specimen is ready, the chromosomes in the cells’ nuclei are examined. If certain genetic disorders are suspected, the genes concerned are identified by special techniques to see whether the foetus is affected by Down’s syndrome or other less common genetic problems.
Some foetal disorders that are not inherited may be discovered by amniocentesis. The amniotic liquid may be analysed
for alpha-fetoprotein and nine out of ten cases of spina bifida can be detected in this way. The degree of foetal anaeimia from Rh iso-immunisation and other disorders of red-cell formation can also be assessed from the amniotic fluid. In preterm labour, amniotic fluid can be examined to see whether the foetal lungs have produced surfactant.
Amniocentesis can’t be performed until
14-16 weeks, which means that the results of genetic tests may not be available until 18-20 weeks. If genetic abnormality is discovered and termination of pregnancy is chosen, it must usually be by induced labour.
Chorionic Villus Sampling (CVS)
This is a method of obtaining foetal cells by passing a slender tube through the cervical canal and using gentle suction to withdraw a fragment of the outer membrane (chorion) of the amniotic sac. CVS may also be performed through the abdominal wall. Both procedures are performed with ultrasound guidance.
More cells are obtained than by amniocentesis, so they can be examined immediately or after a short period of culture. Results are usually available within 10 days, often sooner. CVS can be performed from the eleventh week of pregnancy. The earlier diagnosis of genetic abnormalities allows termination of pregnancy, if necessary, by the safer method of suction aspiration. The test is not suitable for diagnosis of spina bifida. The risk of miscarriage after CVS through the cervix is about three per hundred, and a bit less if performed through the abdomen; less than 1 per cent higher than the risk of miscarriage at 10 weeks among women who haven’t had CVS.
Tests in the future
Researchers in Adelaide have developed a blood test to detect some foetal abnormalities from as early as eight weeks. The new test, which poses no risk for the pregnancy, is performed simply by taking a sample of the mother’s blood and isolating the foetal cells: some cells from the foetal placenta always enter the mother’s blood. The cells may then be cultured and examined for genetic defects. The test is not yet available outside the research centre, but in the future this Australian advance in antenatal diagnosis is expected to be used worldwide.
No doubt in future there will be more and better ways of diagnosing and monitoring foetal health. But it’s important to remember that there are no tests that can tell you for sure that you will have a normal baby.