My DD had an AVSD, diagnosed at our 20 wk U/S. She had both ventricles, it would have seemed like only one atrium, it was a severe atrial septal defect. I just remember it being the scariest time in my life waiting for that official diagnoses, just try to remember that there is almost nothing they can't fix or vastly improve upon this day in age. My dd didn't need surgery until 15 mnths, when she had her open heart and it is hopefully the only one she'll need.
So by one ventricle you could mean the hypoplastic like pp said or a severe ventricular septal defect or something else. Don't go reading on too much about hypoplastic left heart until you have an official diagnoses, I mean there is a lot they can do for it, but don't go googling too much right now, it sounds like your not sure what it is yet. I did some googling even after we got the diagnoses and scared the crap out of myself about AVSD's. Just stick to what the doctor is telling you and I'm sure the next move will be to see a specialist for a prenatal fetal echocardiogram, and they will be able to tell you a lot more. And don't forget that a lot of congenital heart defects can improve in the womb as the heart developes, and at 16wks there is a lot of developing left to do. PM me any time.