First Ultrasound is OK whenever the patient goes to the doctor for first time. But second ultrasound should have been done at 12 or 13 weeks at the time of Double marker. Along with Double marker we do Nuchal Translucency, a USG finding. That will exclude Down's Syndrome to a very large extent.
The USG is done on a number of occasions.
Next Steps
Please ask your doctor about this. Also, for further information and discussion please consult a Gynaecologist on Practo