[Chorionic villus sampling (CVS): level of activity and methods to resolve certain difficulties in interpretation].

As of December 1, 1988, we had, as part of our prenatal diagnostic service, studied 458 transcervical chorionic villus biopsies. Three-fourths of these samples were taken because of advanced maternal age (greater than or equal to 35 years), whereas nearly one fifth were done to alleviate parental anxiety. The remainder were performed because of a precedent chromosomal anomaly in child or parent, to determine fetal sex in the case of X-linked familial disorders, or to obtain DNA for molecular analyses. Among the cytogenetic anomalies detected after 24 to 48 hours of culture, eight involved classical trisomies. In four other instances the chromosomal abnormalities were more difficult to interpret (mosaic trisomies 10, 13 and 15, an apparently uniform trisomy 7). All four were revealed to be "false positives", since neither the amniocenteses nor the karyotypes of the normal newborns (one pregnancy is still ongoing) confirmed an abnormal karyotype. In the case of the trisomy 7 we were able, after birth of the baby, to study two placental biopsies, one of which revealed an abnormality distinct from that detected in the chorionic villi. The observations concerning a fifth false positive are more worrisome, as an apparently uniform trisomy 18, with a fetus showing growth retardation on ultrasound, could not be confirmed in the abortus. Otherwise, we have not encountered a false negative result. In this article we discuss the mechanisms potentially responsible for the cytogenetic discrepancies sometimes observed between fetal and placental tissues. Molecular analyses may help to establish whether a chromosomal anomaly present in fetal chorionic villi had its origin in the pre- or post-zygotic stage; in the latter case the aneuploidy may be uniquely extrafetal.