Most impoant feature to diagnose fetal aneuploidy:

Nuchal translucency evaluation represents the maximum thickness of the subcutaneous translucent area between the skin and soft tissue overlying the fetal spine at the back of the neck . It is measured in the sagittal plane between 11 and 14 weeks. When the nuchal translucency is increased, the risk for fetal aneuploidy and various structural anomalies--including hea defects--is significantly elevated.