Difference in nuchal translucency between monozygotic and dizygotic spontaneously conceived twins

Objective: To assess nuchal translucency (NT) thickness and differences in inter-twin NT thickness among naturally conceived monozygotic and dizygotic twins. Methods: Enrolled women prospectively underwent NT measurement for both twins between 11 and 14 weeks of gestational age. NT thickness and inter-twin NT difference were recorded and compared among dizygotic twins, monozygotic dichorionic twins, and monozygotic monochorionic twins. Results: This study obtained analyzable data for 174 monozygotic twins and 107 dizygotic twins. Of the 174 monozygotic twins, 105 were monochorionic, and 69 were dichorionic. Monozygotic monochorionic twin pregnancies had a significantly higher mean fetal NT thickness (1.48 ± 0.62 mm) than either dizygotic twin pregnancies (1.34 ± 0.50 mm) or monozygotic dichorionic twin pregnancies (1.31 ± 0.48 mm). Mean inter-twin NT difference was significantly lower in monozygotic dichorionic twin pregnancies (0.37 ± 0.19 mm) than in dizygotic twin pregnancies (0.57 ± 0.22 mm) or monochorionic twin pregnancies (0.65 ± 0.43 mm).Irrespective of twin zygosity, NT thickness and inter-twin NT difference were larger in monochorionic twins than in dichorionic twins (1.48 ± 0.62 mm versus 1.33 ± 0.49 mm, and 0.65 ± 0.43 mm versus 0.49 ± 0.23 mm, respectively). Conclusion: During first-trimester ultrasonographic screening for Down syndrome, monochorionic twin pregnancies have the greatest NT thickness and monozygotic dichorionic twin pregnancies have the lowest inter-twin NT difference.