For those who decline diagnostic testing, an evaluation for additional first trimester markers of fetal aneuploidy such as the presence or absence of the nasal bone may assist in patient counseling; however, nasal bone assessment is more challenging in twin pregnancies [58]. Nuchal Translucency Evaluation After the second trimester anatomy survey and fetal echocardiogram, there are no robust evidence-based recommendations for the US follow-up of dichorionic twins with increased nuchal translucency. However, given the risk for intrauterine demise, it is reasonable that serial surveillance of fetal growth and well-being be performed in the third trimester [45,59,60]. US Pregnant Uterus Transvaginal As in singleton pregnancies, transvaginal US can be utilized at 11 to 14 weeks of gestation to assess the nuchal translucency, additional markers of aneuploidy, as well as early fetal anatomy in dichorionic twins. Fetal nuchal translucency measurements are similar as long as the same principles for image acquisition and measurement are employed [29]. Given the larger uterine dimensions from the presence of two fetuses, two sacs of amniotic fluid, and two placentas, a combination of both transabdominal and transvaginal imaging may be optimal to fully assess dichorionic twins with increased nuchal translucency. Variant 4: Increased nuchal translucency in monochorionic twins at 11 to 14 weeks of gestation. US Duplex Doppler Pregnant Uterus The detection of abnormal flow across the tricuspid valve or in the ductus venosus by color and pulsed Doppler increases the likelihood of fetal aneuploidy in monochorionic twins with increased nuchal translucency. In addition, both reversed or absent a-waves in the ductus venosus have been reported to be markers for congenital heart disease and the development of twin-twin transfusion syndrome later in gestation [61,62].
Nuchal Translucency Evaluation at 11 to 14 Weeks Gestational Age. For those who decline diagnostic testing, an evaluation for additional first trimester markers of fetal aneuploidy such as the presence or absence of the nasal bone may assist in patient counseling; however, nasal bone assessment is more challenging in twin pregnancies [58]. Nuchal Translucency Evaluation After the second trimester anatomy survey and fetal echocardiogram, there are no robust evidence-based recommendations for the US follow-up of dichorionic twins with increased nuchal translucency. However, given the risk for intrauterine demise, it is reasonable that serial surveillance of fetal growth and well-being be performed in the third trimester [45,59,60]. US Pregnant Uterus Transvaginal As in singleton pregnancies, transvaginal US can be utilized at 11 to 14 weeks of gestation to assess the nuchal translucency, additional markers of aneuploidy, as well as early fetal anatomy in dichorionic twins. Fetal nuchal translucency measurements are similar as long as the same principles for image acquisition and measurement are employed [29]. Given the larger uterine dimensions from the presence of two fetuses, two sacs of amniotic fluid, and two placentas, a combination of both transabdominal and transvaginal imaging may be optimal to fully assess dichorionic twins with increased nuchal translucency. Variant 4: Increased nuchal translucency in monochorionic twins at 11 to 14 weeks of gestation. US Duplex Doppler Pregnant Uterus The detection of abnormal flow across the tricuspid valve or in the ductus venosus by color and pulsed Doppler increases the likelihood of fetal aneuploidy in monochorionic twins with increased nuchal translucency. In addition, both reversed or absent a-waves in the ductus venosus have been reported to be markers for congenital heart disease and the development of twin-twin transfusion syndrome later in gestation [61,62].