My ob told me that measuring fundal height was a hit and miss practice. He used me for student practice at one point, all three students got the wrong measurement. At one point with one of them I was measuring heaps behind!
He did the same as what em said. He just knew by feel that she was growing, and he said not to worry about the measurements.
As for weigth gain, I didnt gain any weight until I was 35wks. I would be worried if you were losing lots of weight tho. But If you are eating healthy and eating enough to make yourself full, then I wouldnt worry.