A 28-year-old G2P1 woman from Delhi attends the antenatal clinic at 16 weeks of gestation for her routine ultrasound. She reports regular menstrual cycles and is confident of her dates. On transabdominal ultrasound, the crown-rump length (CRL) measures 82 mm, which corresponds to approximately 13 weeks of gestation. The nuchal translucency (NT) measures 2.8 mm. Doppler assessment of the ductus venosus shows an absent or reversed a-wave. There is no other structural anomaly detected. What is the most likely diagnosis?