Fetal aneuploidy screening by non-invasive prenatal testing of maternal plasma DNA sequencing with "false negative" result due to confined placental mosaicism: A case report.

Background: Non-invasive prenatal testing (NIPT) is an accurate screening method with high specificity and sensitivity and a low false-positive rate of trisomy 21, 18, and 13. However, false-negative NIPT results could also limit the clinical application of NIPT.

Methods: A 34-year-old primigravida woman who underwent NIPT at 16 + 3 weeks' gestation was identified as being at high risk for fetal trisomy X (47, XXX). Fetal cardiac defect and hand posture were observed during prenatal ultrasound examination at the 23rd week of gestation. Methods: Amniocentesis conducted at the 24th week of gestation. Fetal karyotyping and FISH identified karyotype 48, XXX, + 18, which indicated that the NIPT failed to detect trisomy 18 in this case. Methods: The couple decided to terminate pregnancy at the 26th week of gestation and was willing to undergo further examinations.

Results: Discordant results between fetus with trisomy 18 and placenta with mosaic T18 were further identified with massive parallel sequencing, which might be due to that the fetal cell-free DNA in maternal plasma for NIPT that was assessed principally originated from the trophoblast cells.

Conclusions: The presence of trisomy 18 mosaicism in the placenta might be the reason for the false-negative NIPT result in this case of double aneuploidy with 48, XXX, + 18, karyotype. Although the NIPT is a valuable screening method that has evident advantages in prenatal aneuploidy screening for certain chromosomal abnormalities compared to other methods, it is not a "diagnostic test" yet.