Placental mosaicism in prenatal diagnosis

Abstract Chromosomal mosaicism is a well-known phenomenon in prenatal cytogenetics and affects approximately 2 % of chorionic villus samples (CVS). The interpretation of mosaicism is challenging, and the major question is whether the abnormal cell line also affects the fetus (true fetal mosaicism, TFM). While mosaicism detected at CVS turns out to be confined to the placenta in the majority of cases, the individual risk of TFM widely varies and needs to be assessed on a case-by-case basis. This article aims to provide an overview on the different types of mosaicism in CVS, the probability of fetal involvement, the laboratory work-up, implications for genetic counselling, and potential effects upon placental function. It is emphasized that understanding placental mosaicism is crucial for the interpretation of results from non-invasive prenatal screening technologies, like NIPT for common aneuploidies, which are based on cell free placental DNA. Finally, we will discuss recent findings of genomic studies which indicate that placental mosaicism extends far beyond classic chromosome aberrations.