I agree with the girls about the measurements being more a worry than a help.
At 24 weeks I measured in at 29.5cm,,,and have been getting much bigger every week since. However I went for an ultrasound on friday just gone, as my OB was quite concerned as the the huge measurements I was having, and bubs is doing great,,,,and only 2 to 2.5 weeks ahead of dates on bone measurements etc. So I think that as the girls said,,,,,if bubs is active, and you are both well,,,then dont pay any attention to the measurements