Cytogenetic analysis in three pregnancies revealed chromosomal mosaicism confined to chorionic villi. They were ascertained in the third trimester by intrauterine growth retardation (IUGR) in otherwise normal fetuses. In case of triple trisomy 6,21,22 and trisomy 16, it was obvious that these findings were most likely restricted to the placenta. These trisomies act as early lethal factors when they occur in the embryo itself. With trisomy 18, however, the interpretation of the cytogenetic finding remains ambiguous. The question arises as to whether an abnormal karyotype may be the cause of placenta insufficiency or is just coincidentally associated.