From what I was told by my doc when I went through this - so long as the baby is growing (even minimally) and the head and abdomen are growing at the same rate then all is ok: I think the main thing that they will be looking for in your next scan is that the fluid levels are still ok and head isn't bigger then the abdomen or vice versa.. Because these can be indications that there is something wrong.
My daughter was spot on growth until 24 weeks and then she started to drop down in centiles until birth.. Although she was born quite small at term her placenta was perfect, and at one point she was in the 9th centile..