Although an increased nuchal translucency may prompt early echocardiography in monochorionic twins, standard second trimester fetal echocardiography is warranted in all monochorionic twins because of the substantial risk of both structural and functional heart abnormalities regardless of the nuchal translucency measurements [59,65]. US Pregnant Uterus Transabdominal First trimester combined screening with nuchal translucency and maternal serum markers is currently a common choice for aneuploidy screening in monochorionic twins [53,54]. In monochorionic twins, each fetus is assumed to have the same risk of aneuploidy equivalent to the maternal age risk of a singleton. Therefore, aneuploidy risk estimates for trisomy 21, 18, and 13 are calculated using the mean nuchal translucency measurement of the twins, usually obtained via transabdominal US. Overall, first trimester combined screening in monochorionic twins provides detection rates of fetal aneuploidy similar to that reported in singletons but with a higher false positive rate [11,53]. A meta-analysis of first trimester combined aneuploidy screening reported a sensitivity of 87.4% for monochorionic twins compared to 86.2% for dichorionic twins [54]. Nuchal translucency measurements have been observed to be higher in monochorionic twins compared to dichorionic twins, which may explain the higher false- positive rate of aneuploidy screening in monochorionic twin pregnancies. In monochorionic twins with increased nuchal translucency thickness, an assessment for additional first trimester markers of fetal aneuploidy, such as the absence of the nasal bone, may be helpful to patients trying to decide whether to undergo invasive diagnostic testing [58]. Increased nuchal translucency thickness in a monochorionic twin may also be an early manifestation of inter- fetal transfusion and early hypervolemic congestion in a recipient twin.
Nuchal Translucency Evaluation at 11 to 14 Weeks Gestational Age. Although an increased nuchal translucency may prompt early echocardiography in monochorionic twins, standard second trimester fetal echocardiography is warranted in all monochorionic twins because of the substantial risk of both structural and functional heart abnormalities regardless of the nuchal translucency measurements [59,65]. US Pregnant Uterus Transabdominal First trimester combined screening with nuchal translucency and maternal serum markers is currently a common choice for aneuploidy screening in monochorionic twins [53,54]. In monochorionic twins, each fetus is assumed to have the same risk of aneuploidy equivalent to the maternal age risk of a singleton. Therefore, aneuploidy risk estimates for trisomy 21, 18, and 13 are calculated using the mean nuchal translucency measurement of the twins, usually obtained via transabdominal US. Overall, first trimester combined screening in monochorionic twins provides detection rates of fetal aneuploidy similar to that reported in singletons but with a higher false positive rate [11,53]. A meta-analysis of first trimester combined aneuploidy screening reported a sensitivity of 87.4% for monochorionic twins compared to 86.2% for dichorionic twins [54]. Nuchal translucency measurements have been observed to be higher in monochorionic twins compared to dichorionic twins, which may explain the higher false- positive rate of aneuploidy screening in monochorionic twin pregnancies. In monochorionic twins with increased nuchal translucency thickness, an assessment for additional first trimester markers of fetal aneuploidy, such as the absence of the nasal bone, may be helpful to patients trying to decide whether to undergo invasive diagnostic testing [58]. Increased nuchal translucency thickness in a monochorionic twin may also be an early manifestation of inter- fetal transfusion and early hypervolemic congestion in a recipient twin.