[Genetic analysis of the false positive trisomy 7 and false negative trisomy 18 by NIPT-PLUS]

To explore the cause of inconsistency between the results of trisomy 7 by expanded non-invasive prenatal testing (NIPT-PLUS) and trisomy 18 by prenatal diagnosis.

A pregnant woman who received genetic counseling at Jiaozuo Maternal and Child Health Care Hospital on July 5, 2020 was selected as the study subject. NIPT-PLUS, systematic ultrasound and interventional prenatal testing were carried out. The middle segment and root of umbilical cord, center and edge of the maternal and fatal surface of the placenta were sampled for the validation by copy number variation sequencing (CNV-seq).

The result of NIPT-PLUS indicated that the fetus has trisomy 7. Systematic ultrasound has shown multiple malformations including atrioventricular septal defect, horseshoe kidney, and rocker-bottom feet. However, QF-PCR, chromosomal karyotyping analysis, and CNV-seq of amniotic fluid samples all showed that the fetus was trisomy 18. Validation using multiple placental samples confirmed that the middle segment of the umbilical cord contains trisomy 18, the center of the placenta contained trisomy 7, and other placental sites were mosaicism for trisomy 7 and trisomy 18. Notably, the ratio of trisomy 18 became lower further away from the umbilical cord.

The false positive results of trisomy 7 and false negative trisomy 18 by NIPT-PLUS was probably due to the existence of placental mosaicism. Strict prenatal diagnosis is required needed aneuploidy is detected by NIPT-PLUS to exclude the influence of placental mosaicisms.