Potential of DNA zygosity tests for non-invasive evaluation of risk of complications in twin pregnancies

Objectives: To evaluate and compare the potential of DNA analysis and ultrasound examination for diagnosis of high-risk and low-risk twin pregnancies. Material and methods: Chorionicity of 42 twin pregnancies was determined by routine high-resolution sonographic examination between 10 and 14 weeks of gestation. Zygosity was analysed in umbilical cord blood samples collected immediately after the birth by genotyping of 22 autosomal short tandem repeats used in human identity testing. Results: Routine ultrasound imaging in the first trimester of twin gestations revealed 21 low-risk dichorionic (50%) and 21 high-risk monochorionic pregnancies (50%). DNA typing of umbilical cord blood showed 23 twin pairs with different genotypes (low-risk dizygotic pregnancies, 55%) and 19 twin pairs with identical genotypes (high-risk monozygotic pregnancies, 45%). We found four pregnancies (10%), which were diagnosed sonographically as monochorionic diamniotic, but were identified as dizygotic in postnatal DNA testing. They constituted 19% of all high-risk monochorionic pregnancies detected by ultrasound imaging. Conclusions: Our results indicate high potential of prenatal DNA testing of zygosity in identification of low-risk and high-risk twin gestations requiring different prenatal care, especially in cases when chorionicity and zygosity cannot be reliably determined by ultrasound examination and as a supplementary test able to detect gestations misdiagnosed as monochorionic, resulting from fusions of dizygotic placentas. In such cases, dizygosity detected prenatally eliminates the need for frequent prenatal visits typical for monochorionic pregnancies. If chorionicity cannot be unequivocally determined and a prenatal DNA test detects monozygotic twins, a more pessimistic variant of monochorionic pregnancy should always be assumed.