Fetal fraction (FF) characterization by zygosity in twin gestation using single nucleotide polymorphism (SNP)- based noninvasive prenatal testing (NIPT) Herman Hedriana1, Samantha Leonard2, Allison Ryan2, Kimberly Martin2
1University of California Davis , El Macero, CA, United States
2Natera Inc, San Carlos, California, United States
Objectives Data regarding comparison of FF in twins to singletons is limited. SNP-based NIPT distinguishes monozygotic (MZ) and dizygotic (DZ) twins with > 99% accuracy and, using informatics-driven SNP interpretation, individual FF in DZ twins can be estimated. The objective of this study was to characterize the FF in DZ and MZ twin pregnancies in comparison to singleton (S) pregnancies using SNP-based NIPT. FF is the basic quality metric for a reliable NIPT, therefore knowledge of twins FF distribution is essential.
Methods Retrospective FF data were collected for SNP-based NIPT performed between 10/1/2017 and 01/31/2018. For the purpose of this descriptive study, FF were collated for MZ, DZ and S pregnancies, including maternal age (MA), gestational age of blood draw (GA) and maternal weight (MW). Pregnancy outcome data were not available as most were ongoing. The individual DZ-FF were summed for comparison with MZ and S. Where appropriate, data evaluation included descriptive statistics, two-sided
t-test and correlation analyses at the 5% significance level. The study falls under our IRB exemption protocol.
Results 35,206 tests were performed: 2,580 twins and 32,626 singletons. GA at blood draw GA was not different between groups (13.5 weeks). However, MW and MA were significantly greater in twins (P<0.001). The table below compares the FF between groups including FF<2.8% as an estimate for no call. The mean individual DZ-FF was 6.46%±2.63% (median 6.00%, range 0.9%-19.60%). The FF difference between DZ fetuses was 1.37%±1.69% (median 8.00%, range 0%-18.40%). However, the percent difference between DZ fetuses was 17.80%±16.09% (median 13.11%, range 0%-94.85%). Twin FF was 20.91% higher than singletons (17.19% higher in MZ and 24.30% higher in DZ, p<0.0001).
Conclusions Twins FF was less than intuitively expected. This characterization offers insight for further outcome studies to elucidate the clinical significance of FF in twins.
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