Slow growth according to ultrasound

I had my first us last week, it was transabdominal (no clue why they didn't do a transvaginal), showed gestational sac and yolk sac no fetal pole. Had another us today, (1 week later) although fetal pole and faint heartbeat is now visible doc says the baby has grown only 2 days worth and I may miscarry. Anyone else know of this happening early and go on to have a healthy baby? I'm guessing I'm between 5-7 weeks.