The association between chromosomal mosaicism observed on chorionic villus sampling (CVS) and poor pregnancy outcome has been well documented. CVS mosaicism usually represents abnormal cell lines confined to the placenta and often involves chromosomal trisomy. Such confined placental mosaicism (CPM) may occur when there is complete dichotomy between a trisomic karyotype in the placenta and a normal diploid fetus or when both diploid and trisomic components are present within the placenta. Gestations involving pure or significant trisomy in placental lineages associated with a diploid fetal karyotype probably result from a trisomic zygote which has lost one copy of the trisomic chromosome in the embryonic progenitor cells during cleavage. Uniparental disomy would be expected to occur in one-third of such cases. Trisomy of chromosome 7, 9, 15, or 16 is most common among the gestations with these dichotomic CPMs. Nine pregnancies with trisomy 16 confined to the placenta were prenatally diagnosed. Pregnancy outcome, levels of trisomic cells in term placentas, and fetal uniparental disomy were studied. Intrauterine growth retardation (IUGR), low birthweight, or fetal death was observed in six of these pregnancies and correlated with high levels of trisomic cells in the term placentas. Four of the five cases of IUGR or fetal death showed fetal uniparental disomy for chromosome 16. One of the infants with maternal uniparental disomy 16 had a significant malformation (imperforate anus). All infants with normal intrauterine growth showed term placentas with low levels of trisomic cells.(ABSTRACT TRUNCATED AT 250 WORDS)