Prospective prenatal investigations on potential uniparental disomy in cases of confined placental trisomy

In most reported cases of uniparental disomy (UPD) associated with confined placental mosaicism (CPM), a high level of mosaicism or a full trisomy was found in chorionic villi. At the time that we started our investigations, it was not quite clear whether fetal UPD also existed in the more frequently occurring low levels of mosaicism. During a 4‐year period, a follow‐up amniocentesis was performed in all cases of mosaic or non‐mosaic trisomy detected in chorionic villus (CV) semi‐direct preparations and suspected to be confined to the placenta. We performed fluorescent in situ hybridization (FISH) on uncultured amniotic fluid cells to differentiate between generalized mosaicism and CPM. We found 29 cases of CPM and we determined the incidence of UPD in 23 of these cases. Normal biparental chromosome contributions were found in 22 cases. In one case, we detected a maternal heterodisomy for chromosome 16. UPD appeared to be a rare phenomenon in the cases of CPM (type I and/or type III) that we encountered in 3958 consecutively investigated CV samples, and is not the cause of the pregnancy complications found in seven out of 23 cases with CPM. © 1998 John Wiley & Sons, Ltd.