I have seen this a few times all being picked up on ultrasound but one. I've seen it 5times 4 of which had no problems but was fine not a single band around any limbs. (I've heard they can tighten around a limb and come out with a band around the arm or leg but not amputate it just be tight) The only other one I have seen was undiagnosed and it was around a finger. The finger was a bit funny looking not as big as the rest but didn't need amputating or any surgery just left it and I guess it would continue to form as bub grew up and be relatively normal. So I wouldn't panick the ones I have seen have being ok. You will find out more I guess and weather you need more scans later on to check the growth of your bubs limbs and things. Obviously there is a risk but it doesn't mean it will cause any problems.