A leading Irish support network has called for safeguards to be put in place after a new study revealed that noninvasive prenatal testing (NIPT) for the screening of chromosomal abnormalities in twin pregnancies showed correct results just 15.4% of the time.
The ability of the popular screening test to correctly predict anomalies was limited and not stable in twin pregnancies, the study found.
The study, published in peer-reviewed journal Molecular Cytogenetics, reported that 13 women pregnant with twins who underwent NIPT tests received positive results for chromosomal disorders including Trisomy18 and Down Syndrome. However, more comprehensive testing – known as foetal karyotyping which looks at the baby’s genome in full – showed that the NIPT test was actually only correct in just 2 of the 13 women.
Vicky Wall of support group, Every Life Counts, described the results of the study as”shocking” and said that most people who had been led to believe that such tests were almost infallible would find the percentage of false positives “truly alarming”.
“Parents are told that these tests are reliable and very accurate – and are often pushed towards abortion on the basis of these results. The results of this study underline the need for safeguards and for medical professionals to exercise caution and to avoid advising parents towards a course of action that may have devastating consequences.
Ms Wall said that Irish maternity services had been rocked by the tragic case of Baby Christopher in the National Maternity Hospital which reached the courts this summer. The little boy was aborted when doctors insisted that tests showed he had Trisomy 18 – a condition they described as a ‘fatal abnormality’. Results of a full karyotype test, only made available after the abortion, showed the baby was perfectly healthy.
“It’s frightening to think that this new study shows that 85% of the results were incorrect for twin pregnancies,” Ms Wall said. “We need to be assured, especially after the tragic case of baby Christoper, that safeguards will be put in place so that parents are not misinformed and not pushed towards abortion.”
For the study, a total of 1,048 women with twin pregnancies were voluntarily prospectively tested by NIPT to screen for chromosomal abnormalities by sequencing cell-free foetal DNA in maternal plasma at the Maternity and Child Health Hospital of Anhui Province in China over a 4-year period, from March 2016 to September 2020,
Overall, 87.5% of the pregnant women were under the age of 35 and most women had no previous risk factors, thus representing the general obstetric population.
Positive NIPT results were confirmed by karyotyping, while negative outcomes were followed up 42 days after delivery.
Among the 13 women with positive NIPT results, there were 2 cases of T21; 1 case of T18; 7 cases of sex chromosome aneuploidy (SCA); 1 case of microdeletion; and 2 cases of microduplication.
Of the 13 cases, only 2 were true-positive cases confirmed by foetal karyotype analysis: 1 case each of T21 and microdeletion.
The remaining 11 high-risk pregnant women were confirmed as false positive by foetal karyotyping. There were no false-negative cases during follow-up.
Studies have shown that NIPT screening cannot completely avoid false-positive results, according to the authors, due to confounders such as confined placental mosaicism, maternal mosaicism, chromosome copy number variation, and maternal tumors. “Therefore, when NIPT results are inconsistent with karyotype results, clinicians should be patient in the process of interpretation,” they said, adding that clinicians should reasonably guide high-risk pregnant women to verify the result by amniocentesis correlated with a detailed Level II or B level ultrasound.
“In summary, NIPT based on high-throughput sequencing is a screening technique rather than a diagnostic technique, and its performance in twin pregnancies is weaker than that in singleton pregnancies,” wrote the authors.