Blood test vs ultrasound???

Hajra
Hi ladies just a quick question I had panorama blood work done at 12 weeks and it all came back normal or low risk for downs but at my 20 weeks ultrasound they saw a shadow on baby's heart, doctor was not concerned at all he said he had to mention it to me but he said he trusts the blood work. I'm still worried. Has anyone had anything like this happend to them???