Confined placental origin of the circulating cell free fetal DNA revealed by a discordant non-invasive prenatal test result in a trisomy 18 pregnancy

Background: Non-invasive prenatal testing (NIPT) by massively parallel sequencing is a useful clinical test for the detection of common fetal aneuploidies. While the accuracy of aneuploidy detection can approach 100%, results discordant with the fetus are occasionally reported. In this study we investigated the basis of a discordant T21 positive and T18 negative NIPT result associated with a T18 fetus confirmed by karyotyping.

Methods: Massively parallel sequencing was used to detect fetal DNA in maternal circulating plasma. The parental origin and nature of the fetal and placental aneuploidies were investigated by quantitative fluorescent PCR of short tandem repeat (STR) sequences and by copy number variation (CNV) sequencing.

Results: There was no evidence of T21 maternal mosaicism, T21 microchimerism or a vanishing twin to explain the discordant NIPT result. However, examination of multiple placental biopsies showed both T21 and T18 mosaicism, including one confined region with a significantly higher proportion of T21 cells. Based on fetal DNA fractions and average mosaicism levels, the effective T21 and T18 fetal DNA fractions should have been sufficient for the detection of both trisomies.

Conclusions: In this pregnancy, we speculate that confined placental region(s) with higher proportions of T21 cells were preferentially releasing fetal DNAs into the maternal circulation. This study highlights placental mosaicism as a significant risk factor for discordant NIPT results.