RESUMO
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.
RESUMO
Due to the high rate of complications, special medical care must be provided especially for monozygotic twin pregnancies, which are characterized as having 2.5 times higher mortality of fetuses. In recent years, examination of cell-free DNA (cfDNA) circulating in maternal plasma has become a useful noninvasive method of prenatal diagnosis. However, fetal DNA constitutes only 3-20% of plasma cfDNA during pregnancy. Short tandem repeats (STRs) are routinely used in forensic examination of DNA mixtures and are able to identify 5% minority components. Haplotypes of deletion/insertion polymorphisms and STRs (DIP-STRs) are able to detect even 0.1% minority components of DNA mixtures. Thus, STRs and DIP-STRs seem to be a perfect tool for detection of fetal alleles in DNA isolated from maternal plasma. Here, we present a novel noninvasive prenatal diagnosis technique of determination of pregnancy zygosity based on examination of feto-maternal microchimerism of plasma cfDNA with the use of STRs and DIP-STRs. Our preliminary results based on 22 STR loci showed 67% sensitivity, 100% specificity and 82% accuracy for prenatal detection of twin dizygosity. The corresponding values for seven DIP-STRs were 13%, 100% and 54%, respectively. Owing to assay performance, low DNA input requirements, low costs (below 10 USD per patient) and simplicity of analysis, genotyping of STR/DIP-STR markers in maternal plasma cfDNA may become a useful supplementary test for noninvasive prenatal diagnosis of twin zygosity in cases when chorionicity and zygosity cannot be reliably determined by ultrasound examination and prognostic value may be provided by a DNA test determining pregnancy zygosity.
