Genetic screening: ethical issues - Nuffield Council on Bioethics

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Procedures for antenatal testing
Amniocentesis
The most widely used technique of prenatal diagnosis, most commonly carried out at
15-18 weeks gestation, although it can be carried out as early as 12 weeks.
Ultrasound is used to locate the placenta, and a small quantity of amniotic fluid, which
contains cells shed by the developing fetus, is withdrawn through a needle from the
amniotic cavity. Cells have to be cultured before chromosome examination (for
example, to detect Down’s syndrome) or DNA analysis can take place. <strong>Genetic</strong>
diagnosis is not usually possible until 16-20 weeks of pregnancy. There is still some
uncertainty about the exact risk to the pregnancy from amniocentesis largely because
the risk is so low that it is extremely difficult to measure. The best studies suggest a
0.5-1% excess risk of spontaneous abortion following amniocentesis at 15-16 weeks
and a slightly increased incidence of mild respiratory problems in the newborn. Good
data are not yet available on the risks of early amniocentesis.
Chorionic villus sampling (CVS)
A procedure whereby a small sample of chorionic (placental) tissue, which shares the
genetic make up of the fetus, is removed for prenatal diagnosis. It is usually performed
at about 10 weeks of pregnancy with only minimal discomfort and often allows a
genetic diagnosis to be achieved before 12 weeks’ gestation. CVS requires first-class
ultrasound and an expert and well-trained team. The risks are higher than for
amniocentesis : an MRC trial gave 2-4% excess miscarriage risk.
Fetal blood and tissue sampling
Fetal blood sampling is used for a variety of purposes : for example, for the diagnosis
of the haemoglobin disorders and haemophilia when DNA diagnosis is not possible,
and for the assessment of rhesus haemolytic disease. It can be performed safely only
after the seventeenth week of pregnancy and only by experts. The initial, highly
specialised technique of fetoscopy has now been replaced by the safer and less
specialised technique of ultrasound-guided transabdominal needle puncture of the fetal
cord insertion. The risk figure for cordocentesis is 2%.
Ultrasound
Ultrasound scanning is now a basic part of obstetric practice. There is a continual
increase in the range and capabilities of the best equipment and a decrease in the size
and cost of basic machines. Many major structural malformations can be detected by
ultrasound in the second trimester of pregnancy (at about 16-20 weeks’ gestation).
There is no evidence for a harmful physical effect of diagnostic obstetric ultrasound.
Its main limitations are the dependence on the skill and experience of the operator and
the quality of the equipment, and its main risk is misinterpretation of the image leading
to failure to detect abnormalities (false negatives) or to abortion of a healthy fetus (false
positives). Another problem is that the natural history of many detectable fetal
anomalies, for example, choroid plexus cysts, renal pelvicalcyceal dilation and
ventricular dilation, is inadequately documented or unpredictable. This makes it difficult
to give advice and can generate unnecessary anxiety.