1 Techniques of chorion villus sampling

Abstract

Whilst randomized studies into the safety of chorionic villus sampling (CVS) are already under way the technique is now offered as an acceptable alternative to amniocentesis in many diagnostic centres. In counselling, the obstetrician can now quote a risk to the pregnancy of 2-4% which, even if the inevitable losses before 16 weeks are excluded, represents probably at least twice the risk of amniocentesis. The evolution of the obstetric procedures has meant that the transcervical approach to CVS has been the most popular to date but there is now increasing interest in transabdominal aspiration as it minimizes the possibility of infection. The two best known transcervical methods are aspiration with a plastic or metal cannula and biopsy with rigid forceps. The majority of aspirations have been performed using the Portex cannula technique. Using this, three centres (Milan, Chicago and Philadelphia) have had experience of over 5000 cases with a failure rate of less than 1% and a minimal fetal loss of 2.2%. However, the proportion of fetal losses may be between 4 and 7% by the time complete obstetric follow-up is available (Brambati et al, 1985). A similar technique has been used with a variety of cannulae. The experience of the first 1000 cases from San Francisco (Hogge et al, 1986) led them to conclude that CVS by this technique was acceptably safe but that continuing investigation was needed before CVS was offered routinely as an alternative to amniocentesis. The only other transcervical technique that is practical for routine use is biopsy with rigid forceps. The failure and fetal loss rates associated with this method are comparable to the aspiration technique. The pioneering work of Hahnemann and his colleague Smidt-Jensen has established transabdominal aspiration as a reliable alternative approach. Its main advantage should be to minimize the risk of infection which is inherent in all transcervical techniques. In addition, it does not need to be confined to 9-11 weeks gestation and thus offers couples the possibility of diagnosis between 12 and 14 weeks rather than waiting for amniocentesis at 16 weeks. The quality and quantity of the sample depends on the size of the needle used and the technique is said to have high patient acceptability. Nevertheless, fetal losses occur following the procedure (1.8-3.2%) and it may not always be successful (failure rate 1.9-4.2%).