Prenatal diagnosis and genetic screening : community and service implications / a report of the Royal College of Physicians.

  • Royal College of Physicians of London.
Date:
1989
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Licence: Attribution-NonCommercial 4.0 International (CC BY-NC 4.0)

Credit: Prenatal diagnosis and genetic screening : community and service implications / a report of the Royal College of Physicians. Source: Wellcome Collection.

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    Fetal blood and tissue sampling Chorionic villus sampling (CVS) but the balance of evidence is against this. The problems identified with amnio centesis represent only a slight statistical increase in recognised complications in the newborn, all are correctable and there is no evidence that they are anything but transient. The figure now commonly quoted to pregnant women is a 0.5-1% risk of miscarriage following the procedure; the exact figure given being influenced by the personal experience of the operator. 2.36 Fetal blood sampling is used for diagnosis of the haemoglobin disorders and haemophilia when DNA diagnosis is not possible, for immunological diagnosis of combined immune deficiency syndromes or intrauterine infections, and for rapid karyotyping of fetal lymphocytes when a malformation has been detected by ultrasound. It can be performed safely only after the 17th week of pregnancy and only by experts. Initially, fetal blood sampling was done by fetoscopy, a highly specialised procedure with a 3-7% risk of fetal loss [16]. This is now being replaced by the safer and less specialised technique of ultrasound-guided transabdominal needle puncture of the fetal cord insertion [17]. Fetal skin and liver biopsies, selective feticide of one affected twin, and intrauterine transfusion may also be performed [18]. 2.37 Chorionic villus sampling is a relatively new procedure whereby a small sample of chorionic (placental) tissue is removed for prenatal diagnosis [19]. CVS can be carried out in the first trimester of pregnancy with only minimal discomfort and often allows a genetic diagnosis to be achieved before 12 weeks’ gestation. This means that termination of pregnancy, when requested, can be carried out simply, painlessly and in privacy under general anaesthesia. First trimester testing may greatly reduce the emotional and some of the ethical conflicts involved in prenatal diagnosis, especially for high-risk pregnancies. 2.38 One of the main dangers ofCVS is that although it appears simple, it requires first-class ultrasound and an expert and well-trained team. Transcervical CVS is most successful and has fewest complications between 9 and 11 weeks of gestation. Transabdominal CVS can be done at any stage of pregnancy provided that the placenta is in an accessible position. As some pregnancies are more suitable for transcervical and some for transabdominal sampling, both techniques are practised at many centres. 2.39 The presence of chorionic villi in the sample taken is confirmed microscopic ally at the bedside. Usually about 20-50 mg of tissue is needed for a genetic diagnosis. If villi are absent or insufficient the procedure is repeated, but no more than twice on any one occasion. Table 7 shows why the time taken for a diagnosis ranges from 3 days to 3 weeks, depending on the laboratory method involved. 2.40 The rapid spread of CVS testing presents a real challenge in evaluation of risk because the high spontaneous abortion rate in the first trimester of pregnancy, particularly among older women, confuses the evaluation of losses due to the procedure. A WHO-sponsored registry showed a total fetal loss rate of less than 4% in over 10,000 cases reported from 1982 to the end of 1986, while some large expert centres report a total fetal loss rate of 2-3%, estimated to be about 1% in excess of expectation at this stage of pregnancy [20]. There is thus already considerable evidence that CVS is a suitably safe procedure for prenatal diagnosis of high-risk genetic disease. However, more information is needed on its short- and long-term risks [21]. The first report from the randomised controlled comparisons ofCVS with amniocentesis, now under way, tend to be reassuring [21 A], These randomised studies also have the important advantage that they generate matched cohorts of children, one exposed to amniocentesis and one to CVS, who can be followed-up and examined in the future should questions arise about possible subtle long-term effects.