Fundal height seems to be pretty unreliable, it can be influenced by the amount of amniotic fluid, not just the size of the baby. I wouldnt be worried if I were you. Just as an example, a week and a half after an ultrasound that dated my baby at 18 weeks, my fundal height was measured at 24 weeks by the midwife, so obviously it cant be a good indicator of baby size. At my last check 2 weeks ago I was measuring a week ahead.