90th percentile is still normal. At 20 weeks I had a scan that had abdo > 99 percentile - so very abnormal. They were looking into investigating GD early, ans some other tests too. I was hesitant, with good cause. I bought the scan images to the doctor- the sonographer was sooo off lol, the measurement was so poor it looked like bubs was wearing a hula hoop lol. Doc still wanted to do tests (measurements were readjusted to 85% ile) but midwife says its just a healthy bub. I wouldnt be too concerned with 90% ile, sounds like they are just being cautious :-)