[False positive non-invasive prenatal testing results due to vanishing twins]

To explore the mechanism and duration of false positive results of non-invasive prenatal testing (NIPT) caused by vanishing twins.

To detect the variation of cell-free fetal DNA fraction before and after the fetal death and explore its influence on the results of NIPT at different gestational weeks. Prenatal diagnosis was also carried out on amniotic fluid sample derived from the survivor twin. After birth, the two placentas and papyraceous fetus were obtained to ascertain the definitive genetic diagnosis and pathological changes through fluorescence in situ hybridization, fluorescence quantitative PCR and histopathological examination. Eight cases of vanishing twins leading to discordant NIPT results were reviewed for determining the duration of this influence.

The vanishing twin has led to immediate flooding of cfDNA into the maternal plasma due to necrotic cytotrophoblasts, which in turn caused increased release of fetal DNA in a short time. However, this did not change the NIPT result for a period of time. The tissue and chorionic villi of perished fetus presented extensive degenerative necrosis.

The false positive NIPT result caused by vanishing twins may be attributed to continuous release of DNA fragments into the maternal plasma by the fetuses. The influence of the vanished fetuses, which may lead to discordant NIPT results, can last for at least 7-8 weeks but no more than 12-14 weeks during the first and second trimester.