Down's Syndrome with only one soft marker?

Alyssa • ✨ Mum² 🎀 🏈 ✨ Fiancé to a keeper 💍
I went for my 20wk "gender reveal" scan last Wednesday, and I got a call from my OBG Friday morning, apparently they saw a small EIF in the baby's heart :( they said that in and of itself, it isn't dangerous, but that it is a soft marker for Down's Syndrome. There were no other visual signs (missing nose bone, thickened back of the neck, shortened limbs, etc.) I'm going back for a Level 2 ultrasound next week, they said that maybe the EIF will have disappeared by itself, and if it hasn't, from that point I'll move to a blood test that's 99.99% accurate.
Has anybody else had this happen, and it came back that your baby DID have Down's with only ONE soft visual marker? I opted earlier on to not have any genetic testing done, I figured we'd do it later if the time came, and I guess it has. Not feeling too great about all of this, and I'm way too far along to even consider terminating, so any advice or related experience is helpful!