My very close friend just had a baby in january with that same issue. of course she was worried throughout the pregnancy, but the frequent dr visits kept her informed and a little more at ease. don't get too worried, because stress can be bad for the baby, too (my last pregnancy was high risk as well, and my stress made it WORSE)...
well, just want to tell you that EVERYTHING with my friend's awesome little baby is 100% ok. he's an extremely healthy, happy, big baby...
so, yes- PRAY and take it easy. And you should get the echocardiogram because there COULD be issues you wouldn't see with the sono, and you NEED to know! Hope all goes well, and I will pray for your little angel, too:)
Lots of hugs for you and your little one!