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Is NIPT reliable when I carry a twin or triplet?
A message from the editor: Prof. Joris Vermeesch, Ph.D.
Twinning is quite frequent. Current estimates indicate the twinning frequency in the US to be about one every 30 pregnancies. Due to fertility treatments, this incidence is about double the natural incidence which is estimated at one every 60 to 80 pregnancies. We can distinguish dizygotic twins, twins derived from two zygotes versus monozygotic twins originating from a single zygote that split during the very early cleavage divisions. The rate of monozygotic twinning is about 35/10000 live births regardless of race, geography or maternal age. The rate of dizygotic twins does vary dependent on geography, maternal age and family histories and can be as low as 30-40 live births in Asian countries to over 180 per 10000 live births in African countries. We know that genetic variation can influence the dizygotic twinning rate. Triplet pregnancies are more exceptional with an incidence of 1 to 2 in 10000 live births.
Despite the relatively high frequency of twin pregnancies, information about NIPT in twins is scanty. For singleton pregnancies test accuracies are widely reported by test providers and are the core of the validation studies with sensitivities and specificities over 98%. With the exception of some test providers, this information is notoriously lacking from most websites. Independent of what the providers post on their website, large scale studies providing rigorous data about test accuracies in twins are lacking. As a consequence, professional societies have often refrained from supporting or even advocating against the use of NIPT for twin pregnancies.
How safe is it to have a NIPT test when having a multiplet pregnancy? Well, the good news is that the available studies, although somewhat underpowered, suggest that the sensitivities and specificities for screening of trisomies 21, 18 and 13 among twin pregnancies are comparable to singleton pregnancies. The amount of circulating fetal DNA is on average higher compared with singletons, but not double. Monozygotic twins are genetically identical and as a consequence the test behaviour is similar compared with singleton pregnancies. The main question pertains to the NIPT performance in dizygotic twins. Although this information may not be available for all NIPT methodologies being used, for the major technologies this information is trickling in and the results look promising. Overall, the performance of cfDNA testing for trisomy 21 in dizygotic twin pregnancies is similar to that reported in singleton pregnancy and is superior to that of the first‐trimester combined test or second‐trimester biochemical testing. One caveat may be that the success rate of the test is slightly lower than for singleton pregnancies and that slightly more secondary sampling is required. While NIPT is mainly aimed for the detection of trisomy 21, other chromosomal anomalies and especially the risks for the presence of a fetal trisomy 13 and 18 are also reported. The incidence of trisomy 13 and 18 is only half of the incidence of chromosome 21. As a consequence, the number of twin cases with trisomies 18 and 13 reported is thus too small for accurate assessment of the predictive performance of the cfDNA test.
As a conclusion, I would say that the stars are properly aligned to assure that twin NIPT is acceptable. Make sure you check with the test provider which information is available. With increasing adoption of NIPT we will obtain better insights about the success rates as well as the accuracies, not only for trisomy 21 detection but also for trisomy 13 and 18. Of course, NIPT is only one of the methods of care during a pregnancy. Ultrasonographic analysis has for the last thirty years proven indispensable and will remain to be so in the years to come. In case of a triplet pregnancy, a good oby/gyn remains the golden standard.