But we had a previous pregnancy in which the baby had a severe heart defect, and she died at 29 wks. They are u/s, just more sophisticated as they can measure blood flow etc. I am surprised they are recommending this to you, since your nt measurement was normal (right?).
The other thing to do would be to get your level II done and if they see anything out of the ordinary then do the fetal echo. I believe you can get your level II done as early as 18 weeks, but I'm not sure.
Also, to be reassuring, most heart defects are very fixable. Our baby was diagnosed with a cardiomyopathy, which is very rare (in which the heart looks normal but doesn't pump right). The amnio was normal so we don't know what caused it.
Given our history, we will probably do one this time but after the level II, probably around 22 weeks. GL