Objective: To determine whether assessment of ductus venosus flow at 11–13 weeks’ gestation improves the detection rate of cardiac defects achieved by screening with nuchal translucency (NT) thickness. Methods: Prospective first-trimester screening for aneuploidies, including measurement of fetal NT and assessment of ductus venosus flow. The performance of different combinations of increased fetal NT and abnormal blood flow in the ductus venosus in screening for major cardiac defects was examined. Results: The study population of euploid fetuses included 85 cases with major cardiac defects and 40,905 with no cardiac defects. The fetal NT was above the 95th and above the 99th centile in 30 (35.3%) and 18 (21.2%) of the fetuses with cardiac defects, respectively, and in 1,956 (4.8%) and 290 (0.7%) of those without cardiac defects, respectively. Reversed a-wave was observed in 24 (28.2%) of the fetuses with cardiac defects and in 856 (2.1%) of those with no cardiac defects. Specialist fetal echocardiography for cases with NT above the 99th centile and those with reversed a-wave, irrespective of NT, would detect 38.8% of major cardiac defects at an overall false- positive rate of 2.7%. Conclusions: Assessment of ductus venosus flow improves the performance of NT screening for cardiac defects.