Belgian genetic centres screen for Down syndrome in 4150 twin pregnancies
A large-scale study by the eight Belgian genetic centres shows that the non-invasive prenatal test (NIPT) is as reliable in twins as it is in singletons when it comes to detecting Down syndrome.
The non-invasive prenatal test (NIPT) assesses a pregnant woman's risk of having a baby with Down syndrome and other chromosomal abnormalities and has been the norm in Belgium for several years now. The test has almost completely replaced the old first-trimester screening, which involved neck fold measurement. Since July 2017, the NIPT has been fully reimbursed by health insurance providers for every pregnant woman, regardless of her previous risk.
While the reliability of the NIPT has been amply demonstrated in singleton pregnancies, more research was needed to verify this reliability in twin pregnancies. As a result, some labs tended to advise against the NIPT in case of twins or warn patients of its supposedly lower reliability. However, there is an urgent need for a reliable and safe test that can estimate the risk of Down syndrome in twin pregnancies.
'The classic first-trimester screening isn't very reliable in twin pregnancies: not only does this test miss over 10% of Down babies, it also incorrectly indicates an increased risk in over 5% of twin pregnancies', says Katrien Janssens (UAntwerp).
In an unprecedented large-scale study, the eight recognised Belgian genetic centres assessed the reliability of NIPT when detecting Down syndrome in twins. The researchers compared the results of 4150 NIPTs in twin pregnancies, including 2716 dizygotic (i.e. fraternal) twin pregnancies.
The best follow-up
Lead researcher Margot Van Riel (KU Leuven): 'In this group, 12 pregnancies were found to have an increased risk of Down syndrome. Amniocentesis was carried out in 11 of those 12, and it confirmed the NIPT result every time. In the 2704 pregnancies where the NIPT hadn’t predicted an increased risk of Down syndrome, not a single Down baby was born. In other words, in thousands of twin pregnancies, the NIPT prediction was never wrong.'
The scientists can state this with reasonable certainty, as all additional genetic testing carried out during pregnancy and after birth takes place at one of the eight participating genetic centres. Having a NIPT carried out by a recognised genetic centre therefore constitutes the best possible follow-up for pregnant women.
The study predicted Down syndrome in 0.44% of all twin pregnancies: this is comparable to the incidence of Down syndrome in singleton pregnancies. Van Riel: 'This shows that, contrary to popular belief, the risk of Down syndrome isn't higher when you're carrying twins.'