M.
How far along are you? Have you had an Amniocentesis or CVS for genetics testing? I know they can't test for everything with those, but I thought the major genetics disorders could be identified that way. I had a daughter with Trisomy 18, she was small from the get-go. Each U/S she kept getting more and more behind in size. At 15 weeks when I went in for the amnio appt, the Perinatologist was able to see on U/S that she had other problems that were related to her disorder.
My understanding is that the farther along you are, the more inaccurate the U/S's are in measuring baby's size. I had one at 26 weeks with my son because my fundal height was measuring 24 weeks. On the U/S he still measured small, but then I had many U/S's at 28-30 (cause I was on hospital bedrest) and he measured fine. When he was born at 30 weeks, 5 days, he was of normal size for his gestational age.
I've seen/heard of many babies born with IUGR, where it wasn't really known what the cause was. All of these kids were normal chromosomally. With known causes, I've heard of preclampsia, problems with the cord, Mom's nutrition (if she's having constant M/S), etc.
I really like the Perinatologists at the UofA - Dr. Kathryn Reed and Dr. James Macuilla. I'd go for a second opinion with one of them to try to find out what's going on!
Best wishes!