i wouldn't worry too much at this stage, especially since bub is only measuring a week or so off - and it's still early days. During my first preg i was always told that my DD was small for dates and when she arrived premmie at 34weeks she was nearly 5 pound (which is a great size for being so early). I recently had my 25week scan with this bub and was told she's measuring 3 weeks off - so this time they are actually a little more concerned (but she also has a heart defect so supposedly it's normal for bubs with that to grow at a much slower rate). There isn't much they can do anyway except give me fortnightly scans to check my placenta is feeding her correctly etc. I'm only a small person, as well as my DD and family members so i'm not expecting big babies anyway.
I'm sure your little munchkin is fine and will either catch up or just be a slightly smaller bub