A study involving more than 1,000 women pregnant with twins has shown the Non-Invasive Prenatal Testing (NIPT) to be effective and safe in detecting Down’s syndrome.

The test is already widely used in singleton pregnancies, with high detection and low false positive rates for three chromosomal conditions – Down’s syndrome, Edward’s syndrome and Patau’s syndrome.

Researchers at the Twins Trust Centre for Research and Clinical Excellence at St George’s thought there was potential for NIPT to be useful in twin pregnancies, as it may reduce the need for invasive testing which carries the risk of losing one or more babies.

This research study, which took place in six fetal medicine centres, evaluated whether the IONA test, a form of NIPT using a blood test taken from the mother, could accurately detect these three chromosomal conditions.

For Down’s syndrome, this test was shown to be the most accurate and effective screening test available in twins. It may be slightly less accurate for the other two conditions and therefore not recommended.

The study recommends that the IONA test should be the primary test offered for Down’s syndrome screening in twins.

Professor Asma Khalil who led the research team said: “We know that non-invasive prenatal testing (NIPT) is the most accurate screening test in twin, as the case in singleton, pregnancies. The potential advantages of NIPT in twin pregnancies are even greater than in singletons, in particular lower need for invasive testing and consequent fetal loss rate of the other healthy twin.

“However, several organisations do not recommend this test in twin pregnancies and called for larger prospective studies which we have now successfully completed. To have published a multicentre study during the current pandemic – and in the number one journal for our specialty – is a real achievement for everyone involved.”

Keith Reed, CEO of Twin Trust: “This is an important study with a clear recommendation.

“For this to take place during the Covid pandemic is testament to Asma and her team and demonstrates the importance fellow clinicians placed on the research.

“National and international organisations should now consider updating their guidelines on the implementation of NIPT in twin pregnancies and twin parents should be made aware of the test as and when necessary.”

Diagram: Non-invasive prenatal testing (NIPT) using cell-free DNA in dizygotic (non-identical) twin compared to singleton pregnancies

During pregnancy, cell-free DNA (short DNA fragments) from the mother and fetus circulate in the mother’s blood. In singleton pregnancies where the fetus is affected by Down’s syndrome, NIPT would detect slightly more chromosome 21-specific DNA in the blood.

In dizygotic (non-identical) twins, cell-free DNA from both twins will be circulating in the mother’s blood. If one of the fetuses is affected by Down’s syndrome, there will be less chromosome 21-specific DNA in the blood than in singleton pregnancies, even when the fetal fraction (proportion of cell-free DNA in the mother’s blood) is the same for both twins.

Notes to editors

The full article has been published in the American Journal of Obstetrics and Gynaecology. Non-invasive Pre-natal Screening in Twin Pregnancies with cell-free DNA using the IONA Test: a prospective multicentre study https://www.ajog.org/article/S0002-9378(21)00028-4/fulltext (Jan 2021)

Participants in the study were recruited before the Covid-19 outbreak started, while the analysis of data and writing of the paper was carried out during the pandemic.