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Analysis of cell-free (cf) DNA from maternal blood provides effective screening for trisomy (T) 21 in singleton pregnancies. Data on cfDNA screening in twins are promising though limited. We sought to evaluate the screening performance of cfDNA for T21 in twin pregnancies in a large, diverse cohort. A secondary aim was to evaluate screening performance for T18 and T13.
Retrospective cohort study of twin pregnancies from 17 centers with cfDNA screening performed from 12/2011-2/2020 by one lab using a MPSS approach. Medical record review was undertaken for all newborns including birth outcome, the presence of any congenital abnormalities, phenotypic appearance at birth and any chromosomal testing undertaken in the antenatal or postnatal period. Cases with a possible fetal chromosomal abnormality with no genetic test results were reviewed by a committee of MFM geneticists. Cases with a vanishing twin(VT) and inadequate follow up information were excluded. A minimum of 35 confirmed cases of T21 was required to capture a sensitivity of at least 90% with a prevalence of at least 1.9% with 80% power. Test characteristics were calculated for each outcome.
A total of 1771 samples were sent for twin cfDNA screening. Sixty (3.4%) had a non-reportable result. 27 of 42(64%) cases with a second draw received a result. 72 cases with a VT and 247 cases with inadequate follow up were excluded leaving a total of 1452 cases for inclusion in the analysis. The median maternal age was 35, the median gestational age was 12 weeks, the majority of twins were dichorionic and 42% had IVF(Table 1). Of the 42 T21 cases, 41 were true positives (TPs) and there were no false positives (FPs), for a sensitivity of 97.7% (95%CI: 84.2-99.7). There was one false negative (FN) case resulting in a FN rate of 0.06% (95%CI: 0.08-0.40) (Table 2). All 10 of the T18 cases were detected and there was one FP. Four of the 5 T13 cases were detected.
Cf DNA screening is effective in the detection of T21 in twin pregnancies. Data regarding screening performance for T18 and T13 are encouraging but limited.