A pregnancy with discordant fetal and placental chromosome 18 aneuploidies revealed by invasive and noninvasive prenatal diagnosis.

This study investigated a pregnancy where the fetus was diagnosed with monosomy 18p by invasive amniocentesis and karyotyping. Additional noninvasive prenatal diagnosis, which detects fetal chromosome abnormalities in the circulating cell-free plasma DNA originating from the placenta revealed a related 18p monosomy/18q trisomy, suggesting confined placental mosaicism. Based on recent observations of chromosomal instability in the early preimplantation embryo, this study speculates on the possible embryonic origin(s) of these related but discordant chromosome 18 aneuploidies in the placental and fetal tissues. The findings highlight the potential for both false-positive and -negative noninvasive prenatal diagnosis results in pregnancies where there is either confined placental mosaicism or placental mosaicism. The study investigated a pregnancy involving a fetus with a chromosome disease syndrome called monosomy 18p where part of the short arm of chromosome 18 was missing in the fetal tissues. Using non-invasive prenatal diagnosis which detects fetal chromosome abnormalities in the circulating cell free plasma DNA originating from the placenta, we also detected monosomy 18p as well a related chromosome 18 abnormality involving duplication of the long arm of chromosome 18. This suggested confined placental mosaicism where the constitution of the chromosomes are different between fetal and placental tissues. We speculated that these related chromosome 18 abnormalities arose during preimplantation embryo development, leading to the formation of different chromosome abnormalities observed in the placental and fetal tissues of this pregnancy. Our findings highlight the potential for both false positive and negative non-invasive prenatal diagnosis test results in pregnancies where there is confined placental mosaicism.