What does it mean to have normal range of amniotic fluid index, but at the upper limit of 22cm (33 weeks). what causes this?

Worth following: The upper limit of normal is still normal, however it is worth following up to make sure in a few weeks the fluid volume has not increased. In at least 1/3 of cases we cannot figure out what causes the fluid volume to be high. It can also be caused by gestational diabetes. Rarely a fetal abnormality that reduces the fetal ability to swallow can cause increased amniotic fluid.