Fundal height normally correlates (in cms) with gestational age in weeks (from 16 to 36 weeks). The possible reason for increased fundal height could be : Polyhyramnios, Fibroids, Twin pregnancy, H. Mole, Big baby (Fetal macrosomia) Normal previous USGs rules out fibroid, twin pregnancy and H. Mole. According the above given case scenario the most probable cause is polyhydramnios (as the patient is a known case of gestational diabetes which is a risk factor for polyhydramnios).