I've read in my books about a shadow in the intestines and it is due to the intestines got tangled up (by nature, no one's fault). Due to the tangle, afew things could happen:
1. Restricts food/fluid down the intestinal tract, causing bloating and possibly damaging other vital organs as a result
2. Tangle becomes so bad that blood supply to the intestines is cut off and the intestines die. Baby will need to undergo emergency surgery to remove dead intestine and will need to be on TPN for very long time (could be for rest of lifetime)
But it could also be a one off thing (just like having constipation) and it clears off on it's own after a while (maybe that's why they want to do another scan 2 weeks later).
Is the doctor a reputable one at a hospital with better equipment? Cos older ultrasound machines may not be able to allow the doctor to check accurately on the problem.