OBJECTIVETo explore the limitation of non-invasive prenatal testing (NIPT) technique through analyzing two false negative cases.

METHODSChromosomal karyotyping analysis was performed on umbilical cord blood sample derived from case 1 at 24 weeks' gestation and peripheral blood sample derived from the neonate of case 2. Placental tissues of case 1 and peripheral blood sample of case 2 were also analyzed by high-throughput sequencing for copy number variations (CNVs).

RESULTSFor case 1, analysis of fetal umbilical cord blood sample showed a translocation type of trisomy 21, i.e., 46,XY,der(21;21)(q10;q10),+21. There were no obvious abnormalities detected at or near the center of the fetal surface and matrix surface of the placenta. High-thoroughput sequencing showed Chr13:(33 840 001 - 115 100 000)×3[60%]/46,XY[40%] at the edge of the placenta, Chr13:(34 080 001-115100000)×3[54%]/46,XY[46%] at the edge of placenta matrix surface, and trisomy 21 in the umbilical cord tissue. For case 2, analysis of the neonatal peripheral blood sample showed a karyotype of 46,XY,del(18)(q22), which revealed a microdeletion in chromosome 18. High-throughput sequencing of the maternal peripheral blood sample stored during pregnancy confirmed it to be chr18: (62 910 000 - 78 020 000)×1 with 15.1 Mb deletion in the fetus. The neonate was therefore diagnosed with partial monosomy of chromosome 18.

CONCLUSIONFalse negative results of NIPT are related with the fraction of circulating cell-free fetal DNA in the maternal serum. NIPT has limitations in detecting fetal chromosomal microdeletion and confined placenta mosaicisms. Routine ultrasound scan is necessary for pregnant women with low-risk indicated by NIPT.