Objective To determine the expression of aquaporin-1,3,8,9 mRNA (AQP-1,3,8, 9) in amniotic membranes in pregnant women with polyhydramnios. Methods Amniotic membranes were collected from women who presented with either polyhydramnios (n= 5)or normal amniotic fluid volume (control, n= 5) underwent elective cesarean sections at term. The AQP-1,3,8,9 mRNA expression were detected by reverse transcriptase-polymerase chain reaction (RT-PCR). Results The expression of AQP-9 mRNA on fetal membranes were significantly higher in polyhydramnios groups (1. 1403±0. 0831) than that of control (0. 5903±0. 1909) (P = 0. 002), although the expression of AQP-1, 3 and 8 showed no significant difference between the two groups (P= 0. 972, 0. 242,0. 608, respectively). Conclusions AQP-9 may play an important role in maintaining the volume and the balance of different components of amniotic fluid in polyhydramnios cases.