SNP Approach to Determine Twin Zygosity From Maternal Blood

Researchers from Fudan University and elsewhere have developed a SNP-based method to determine zygosity and fetal fraction among twin pregnancies from maternal blood. They note that chorionicity, or the number of placentas in a multiple gestation, is a key risk factor of adverse outcomes in twin pregnancies and that establishing zygosity is a step toward determining chorionicity, as nearly all dizygotic twin pregnancies are dichorionic and about 75 percent of monozygotic twins are monochorionic. As they report in the Journal of Molecular Diagnostics, the Fudan-led team developed a noninvasive prenatal screening approach that relies on a binary analysis of the number and allelic fraction of fetus-specific SNPs to determine zygosity from maternal cell-free DNA samples. When they tested their approach in a cohort of 215 singleton, 90 confirmed dizygotic twin, and 18 confirmed monozygotic twin pregnancies, the researchers found that all 90 dizygotic twin pregnancies were identified with a sensitivity of 100 percent and specificity of 100 percent. They additionally used their approach to monitor fetal fraction in nine dizygotic twin pregnancies following a reduction procedure to find subsequent variability in cfDNA dynamics, suggesting caution in the test’s use following a reduction.