These include but are not limited to previous fetus or child with a congenital, genetic, or chromosomal anomaly; known or suspected fetal abnormality detected by US in the current pregnancy; and fetus at increased risk for a congenital anomaly based on the following: 35 years of age or older at delivery, maternal pregestational diabetes, pregnancy conceived via in vitro fertilization, multiple gestations, teratogen exposure, enlarged NT, positive screening test results for aneuploidy including cell-free DNA screening and serum only, or combined first trimester screening [15,16]. US Pregnant Uterus Biophysical Profile BPP is performed in the second and third trimesters, and there is no role for this in the first trimester. As in singleton pregnancies, increased NT measurements are associated with an increased risk of aneuploidy and structural anomalies; markedly increased NT is associated with a greater risk of subsequent demise [33]. In MC twin pairs, markedly discordant NT can be a marker for early-onset TTTS [37]. However, normal fetal anatomy and karyotype were the most common outcomes among MC diamniotic twins with discordant NTs [38]. Some studies of MC diamniotic twins have shown that NT discordance is more predictive of adverse fetal outcomes than CRL discordance [39]. Others, including Allaf et al [40], found that NT and CRL discordances were not predictive of overall adverse outcomes in MC diamniotic twin pregnancies, although this varies with the severity of discordance. Both parameters (NT and CRL discrepancy) had high negative predictive value; thus, the absence of discordance is reassuring [39]. US Pregnant Uterus Transvaginal As with transabdominal US of the pregnant uterus, a next TVUS study in the first trimester can be performed for confirming cardiac motion, demonstrating the intertwin membrane (if not previously visualized) and NT measurements.
Multiple Gestations. These include but are not limited to previous fetus or child with a congenital, genetic, or chromosomal anomaly; known or suspected fetal abnormality detected by US in the current pregnancy; and fetus at increased risk for a congenital anomaly based on the following: 35 years of age or older at delivery, maternal pregestational diabetes, pregnancy conceived via in vitro fertilization, multiple gestations, teratogen exposure, enlarged NT, positive screening test results for aneuploidy including cell-free DNA screening and serum only, or combined first trimester screening [15,16]. US Pregnant Uterus Biophysical Profile BPP is performed in the second and third trimesters, and there is no role for this in the first trimester. As in singleton pregnancies, increased NT measurements are associated with an increased risk of aneuploidy and structural anomalies; markedly increased NT is associated with a greater risk of subsequent demise [33]. In MC twin pairs, markedly discordant NT can be a marker for early-onset TTTS [37]. However, normal fetal anatomy and karyotype were the most common outcomes among MC diamniotic twins with discordant NTs [38]. Some studies of MC diamniotic twins have shown that NT discordance is more predictive of adverse fetal outcomes than CRL discordance [39]. Others, including Allaf et al [40], found that NT and CRL discordances were not predictive of overall adverse outcomes in MC diamniotic twin pregnancies, although this varies with the severity of discordance. Both parameters (NT and CRL discrepancy) had high negative predictive value; thus, the absence of discordance is reassuring [39]. US Pregnant Uterus Transvaginal As with transabdominal US of the pregnant uterus, a next TVUS study in the first trimester can be performed for confirming cardiac motion, demonstrating the intertwin membrane (if not previously visualized) and NT measurements.