Our 9 year old bio daughter was born at 30 weeks. She did a tremendous job of growing up until 8 months of age. Between 8 months and 1 year of age she gained exactly 2 ounces. We saw the pedi endocrinologist, had bone scans done, saw the pedi neurologist, etc. Looking back the best guess anyone could make is that genetics was taking over
My husband is a small man and now that we have 3 bio kids, we have seen the same pattern in them, not as drastic, but same overall: growing well and then come to a screeching halt and drop off on the growth charts. Our middle child was a 9 lb 8 oz giant, 100% in weight, height, and head for the first 6 months, by the time he was a year, he was at the 50th for weight and height...still has a big head though.
Get things checked out, but try not to worry too much. Some kids just don't do things the way the charts say they should.