A blood sample is taken from the woman, usually at about 16 weeks in pregnancy. The test involves measuring the levels of three substances, or 'markers', in the
mother's blood: these are alpha- fetoprotein (AFP), unconjugated oestriol (uE3) and human chorionic gonadotrophin (hCG). (A fourth marker is to be introduced soon which may increase the detection rate further.)
Low levels of AFP and uE3, and higher levels of hCG, can indicate a higher risk of a baby with Down's syndrome; high levels of AFP may indicate an increased risk of open neural tube defects, such as spina bifida.
Results are combined with the mother's age to give an individual risk factor. Women who have a risk of 1 in 250 or more are called 'screen positive' and offered amniocentesis to find out if the baby is abnormal. Those whose risk is less than 1 in 250 are 'screen negative'.
Screen positive women may first be given an ultrasound scan to check for dates, since the levels of different markers in the blood vary as the pregnancy advances.
For more information telephone the Ante Natal Screening Service, St Bartholomew's Hospital, 071-982 6293.
Other biochemical screening tests are often referred to as the 'multi-marker', 'double' and 'B- triple'.
The TRIPLE PLUS test is only offered privately by St James's University Hospital, Leeds. It is thought to have a detection rate of 80 per cent and can be carried out at 13 weeks, although the ideal time for spina bifida detection is 16-18 weeks. It can indicate the risk not only of Down's syndrome and neural tube defects, but for other less common abnormalities.
The triple plus measures levels of AFP and uE3, as well as free beta HCG, a breakdown product which makes it possible for the test to be carried out earlier than the triple test. In addition, it measures a fourth biochemical marker for Down's syndrome - neutrophil alkaline phosphatase (NAP). Reading levels of NAP is labour intensive, which is why this test is not yet available on the NHS, although doctors are hoping to develop a simpler, cheaper method of analysis.
The triple plus costs pounds 88. For more information, telephone the Down's Syndrome Screening Service, University of Leeds, 0532 344013.
The NUCHAL TRANSLUCENCY SCAN was developed at the Harris Birthright Research Centre for Foetal Medicine, at King's College Hospital, London. An ultrasound scan which can be carried out as early as 11 weeks' gestation is thought to be able to detect about 85 per cent of foetuses with Down's syndrome and other chromosomal abnormalities.
During the scan, doctors measure the black 'space' behind the neck of the foetus. A large space, of 3mm or more, may indicate the presence of a nuchal membrane associated with abnormality. It may also indicate heart conditions and other foetal problems. The result of the scan is computed with the woman's age to give her an individual risk factor. At the Harris Centre, anyone thought to be at higher risk is immediately offered chorionic villus sampling, an invasive test, with preliminary results available within a week.
The nuchal scan is still part of a research project. Doctors at King's say that although their findings are based on a small retrospective study of 1,200 women, the data holds up in 6,000 cases. Several other hospitals now offer the nuchal scan. For more information, telephone the Harris Centre on 071-326 3040.
AMNIOCENTESIS is normally carried out at about 16 weeks' gestation and can detect chromosomal abnormalities and open neural tube defects. Under local anaesthetic, and using ultrasound, a fine needle is put through the abdomen into the womb to remove some of the amniotic fluid surrounding the foetus. The procedure takes between 10 and 20 minutes and can be uncomfortable.
The fluid contains foetal cells which can then be cultured and examined in the laboratory. Results usually take three or four weeks, which means a woman may be 20 weeks or more pregnant before she is told the result.
Amniocentesis carries a risk of miscarriage, estimated at 1 in 100- 200, although this varies depending on the skill of the operator. It may also result in bleeding or leakage of amniotic fluid.
Because of the risk of miscarriage, it is usually only offered to those at higher risk of having an abnormal foetus.
Some hospitals now offer amniocentesis at an earlier stage in pregnancy, although a recent trial of 2,000 women at the Harris Centre found that early amniocentesis had a 3 per cent miscarriage rate. There are also fears that removing amniotic fluid at an earlier stage may affect lung development.
CHORIONIC VILLUS SAMPLING (CVS) is usually performed between 10 and 12 weeks. Doctors stopped doing it at an earlier stage after evidence emerged linking CVS in the first nine weeks of pregnancy with foetal limb defects. It is used to detect chromosomal abnormalities but it cannot diagnose open neural tube defects.
The test involves removing cells from the chorionic villi - fingers of tissue forming part of the placenta. There are two methods of doing this. A needle is introduced, with ultrasound guidance, into the womb through the abdomen; or a sample is removed by forceps or catheter through the cervix. The procedure can cause discomfort.
Results can sometimes be available within a few days.
CVS has been linked to higher rates of miscarriage than amniocentesis, although now that it has become more established, the risk of both procedures is thought to be similar.