Fundal height measurements is just one tool midwives use in estimating fetal size and it is perfectly acceptable for this measurement to be out by about 3cm. I appreciate you are a little further out than this but just to emphasise you're not that far over normal. Hopefully your scan will help put your mind at ease.
If you are concerned about your own weight then you could try eating full fat dairy, nuts, avocado, etc and try to have 5 smaller meals a day rather that 3 large ones. Please don't worry too much though, if it helps at all I put on no weight until 23 weeks gestation with my twins and I still managed to grow them both to around 3kg at term.