Mosaicism in chorionic villus sampling

Although chromosomal mosaicism is encountered frequently in CVS, it is most often restricted to the extraembryonic tissues. Counseling before CVS should include a discussion of the frequency and significance of placental mosaicism. Patients seeking information about the procedure need to be aware of the slight but not negligible possibility of the need for additional follow-up studies. Additional studies may involve additional risks with emotional and psychological sequelae. Ultimately, no prenatal procedure or combination of procedures can provide 100% accuracy in excluding mosaicism once the issue has been raised. When the mosaicism is felt to be confined to the placenta, counseling should probably address the potential increased risk for pregnancy complication, such as miscarriage and IUGR, with appropriate follow-up of the pregnancy, especially in the third trimester. Excluding the issue of mosaicism, the predictive value of cytogenetic diagnosis after CVS is equivalent to that of amniocentesis. In addition, CVS provides this information at an earlier time in the pregnancy than does amniocentesis. The majority of patients electing CVS (98% or more) will not have to deal with these complicated issues. It is hoped that an increasing cumulative experience with CVS mosaicism will help differentiate low- from high-risk situations. At the present time, however, there are insufficient and inconsistent data with which to accurately assess the risks associated with placental mosaicism. Most of the available literature concludes that additional, more focused studies, including long-term follow-up, are needed to help elucidate a better understanding of these issues.