Monocentric Study on the Performance of Noninvasive Prenatal Testing on Cell-Free DNA for the Detection of Monosomy X.

Objective: Increasing the PPV of monosomy X detected by the non-invasive prenatal test (NIPT) by discriminating a (mosaic) monosomy X genotype of fetal versus maternal origin.

Methods: Out of 30,700 women referred for NIPT between January 2014 and December 2021, 79 had a high risk result for 45,X. Discrimination between fetal and maternal 45,X was made based on the values for ffX, ffY and SeqFF. Follow-up was provided through analysis of amniotic fluid, maternal blood, umbilical cord blood, neonatal blood and/or placental biopsies.

Results: Follow-up data were available for 70/79 women; after exclusion of one twin pregnancy, 69 pregnancies were evaluated (87.3%). Forty one of those were correctly predicted as being maternal or fetal, for an overall PPV of 59.4% (95% confidence interval [CI] 47%-71%). Of the 33 predicted fetal cases with follow-up, 11 were indeed of fetal origin, equating to a PPV of 33.3% (95% CI 18%-52%); three additional cases turned out to be placental in origin, six were maternal and for 13, no explanation could be found. The PPV of maternal cases was 88.2% (30/34 cases with follow-up correctly predicted; 95% CI 73%-97%). One case turned out to be fetal; for the other three, follow-up studies failed to prove the presence of monosomy X. Two cases for which no prediction on the origin of the monosomy X could be made (inconclusive high-risk NIPT result) turned out to have confined placental mosaicism.

Conclusion: Although the PPV for fetal monosomy X remains lower than for the common trisomies, the total PPV for 45,X screening with NIPT can be improved by discerning fetal from (mosaic) maternal 45,X genotype. Thorough follow-up to determine the origin of the aberrant NIPT result is advised, so that women can be adequately counseled on the risk in their current and future pregnancies and, in case of maternal mosaic monosomy X, of their own prospects.