It's easy to advise not to worry
But be calm .,as it will have an effect on the fetus .
You have not mentioned the N.T . thickness .
Next Steps
At 20-22 weeks , anamoly scan is done which rules out your suspicion .
Health Tips
In your case it's better to do a triple or quadruple test . this test plus anamoly scan report either confirms or rules out Trisomy 18.