Vanishing twin syndrome affects NIPT results

Noninvasive prenatal testing (NIPT) analyzing cell-free DNA (cfDNA) in maternal plasma may be used for fetal Rhesus D (RhD) status assessment when there is a high risk for Rh incompatibility and for fetal sex determination. As highlighted by a report published in Prenatal Diagnosis describing findings from serial ultrasound and cfDNA testing, clinicians should be aware of the potential for confounding of NIPT results by cfDNA released from a vanishing twin.

In the case that was the subject for the report, cfDNA from the lost twin was present in the mother’s circulation for at least 9 weeks after its demise. Noting that the incidence of the vanishing twin syndrome in natural pregnancies is probably underestimated, the authors stated that the case reinforces the value of early ultrasound monitoring of pregnancies where NIPT is planned in order to avoid misinterpretation of the results because of a vanishing twin.

The case involved a 37-year old alloimmunized RhD-negative woman who delivered an Rh-positive baby girl by cesarean section at 34 weeks gestation. The woman had a history of five previous pregnancies and had been offered NIPT because her history included a pregnancy involving a female stillborn with erythroblastosis fetalis and a normal, RhD-negative boy.

Ultrasound monitoring initiated at 6.6 weeks of gestation revealed the presence of twins, but a beating heart was no longer detectable on ultrasonography at 7.6 weeks. The gestational sac of the lost twin remained evident in ultrasounds performed at 12 and 14.2 weeks, but was no longer seen at 16.6 weeks of gestation, at which time the surviving fetus was determined to be female based on ultrasound.

Rh antibody titers were measured in maternal blood collected every 3 to 4 weeks beginning at 8 weeks of gestation and were stable at around 1:64. NIPT was performed at 14.2 and 16.6 weeks of gestation and about every 4 weeks thereafter through week 30 for RhD genotyping and fetal sex determination. The RhD results were consistently positive and corresponded with results of Coombs testing and serology at birth. There was discordance, however, between NIPT and ultrasound for fetal sex at gestational weeks 14.2 and 16.6 when Y-chromosome sequences were detected in cfDNA analyses conducted with two different assays. Y-chromosome sequences were no longer detected in cfDNA analyses conducted at weeks 21, 25, and 30.

Discussing the case, the authors noted that previous reports describe the potential for erroneous results with NIPT because of the release of cfDNA from the trophoblast of the perished embryo. However, the presence of a vanishing twin as the cause for conflicting results between NIPT fetal sexing and ultrasonography was only hypothesized in the prior cases. In contrast, frequent monitoring with ultrasound and cfDNA testing in the present case provided evidence to support such a hypothesis by demonstrating that the positive and negative Y-chromosome sequence findings corresponded with the ultrasonographic status of the vanished twin’s gestational sac.