The study in the journal Molecular cytogenetics found that the combined PPV was only 15.4% among the 13 twin pregnancies with positive NIPT results.
“The superior performance of NIPT in single pregnancies is recognized as an almost perfect screening method, giving the medical community and the public hope that the testing technology may be applicable to pregnant women with twin pregnancies as well.” , wrote the authors.
Recent studies of NIPT in twin pregnancies have primarily evaluated trisomy 21 (T21), trisomy 18 (T18) and trisomy 13 (T13), according to the authors, but the use of NIPT for sex chromosome diseases , microdeletions and microduplications has not been systematically evaluated.
“This is the first prospective study in which NIPT has simultaneously detected sex chromosome autosomes and aneuploidy, microdeletions and microduplications as well as in twin pregnancies,” they wrote.
Over a period of 4 years, from March 2016 to September 2020, a total of 1,048 twin pregnant women were voluntarily tested prospectively by the NIPT for chromosomal abnormalities by sequencing of the cell-free fetal DNA (cffDNA) in maternal plasma. at the Maternity and Child Health Hospital of Anhui Province, China.
Overall, 87.5% of pregnant women were under 35 and most women had no previous risk factors, thus representing the general obstetric population.
Positive NIPT results were confirmed by karyotype, while negative results were followed 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 analysis of the fetal karyotype: 1 case each of T21 and microdeletion.
The remaining 11 high-risk pregnant women were confirmed as false positive by fetal karyotyping. There were no false negative cases during follow-up.
The combined PPV of DPNI for screening for T21 and T18 was 33.3%, while the PPV for T21 was 50%.
The authors noted that NIPT assesses free placental DNA, not fetal DNA, and that placental mosaicism compromises its performance in screening for fetal chromosomal abnormalities; therefore, it cannot be absolutely confirmed that the cause of the false positives in the study is limited to confined placental mosaicism.
Studies have shown that NIPT screening cannot completely avoid false positive results, the authors say, due to confounding factors such as confined placental mosaicism, maternal mosaicism, variation in chromosome copy number and tumors. kindergartens. “Therefore, when the NIPT results are inconsistent with the 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 puncture of the amniotic fluid with the Z value, cffDNA concentration and ultrasound B.
“In summary, NIPT based on high throughput sequencing is a screening technique rather than a diagnostic technique, and its performance in twin pregnancies is poorer than in single pregnancies,” the authors wrote.
- Cheng Y, Lu X, Tang J, et al. Performing non-invasive prenatal tests for fetal chromosome abnormalities in 1048 twin pregnancies, Mol Cytogenet. Jun 30, 2021; 14 (1): 32. doi: 10.1186 / s13039-021-00551-4.