Imprinted genes play an important role in placental and embryonic development. Abnormalities in their regulation can result in placental and embryonic dysplasia, leading to congenital diseases. The imprinting state, expression, and function of aquaporin-1 (Aqp1) were explored in knockout mice by imprinting analysis, real-time PCR, and immunohistochemistry. In the present study, Aqp1 was identified as a new, imprinted, and placenta-specific maternally expressed gene in the mouse. Compared with wild-type Aqp1(+/+) mice, there was significant placental and embryonic overgrowth in Aqp1(-/+) (loss of maternal allele) and Aqp1(-/-) mice, but not in Aqp1(+/-) (loss of paternal allele) mice at Embryonic Day (E) 12.5-E18.5. In addition, the masses of Postnatal Day 0 (P0) embryos (Aqp1(-/-) and Aqp1(-/+)) were highest among the four types. In Aqp1(-/+) and Aqp1(-/-) mice, phenotypic analysis indicated that the number and branching of blood vessels, as well as the labyrinth area, increased significantly in placentae of E12.5-E18.5 mice. Moreover, there were abnormalities in the placental junctional zone and the labyrinthine zone at E15.5. Quantitative analysis showed that Aqp1 expression decreased significantly in the placentae of Aqp1(-/+) and Aqp1(-/-) mice at E15.5, and that the AQP1 protein expression signals were detected weakly in the decidual and spongioblast layers. Our results demonstrate that Aqp1 is maternally expressed in the placenta, and that its deficiency resulted in placental abnormalities in the mouse. Aqp1 may have a specific inhibitory role in mouse placental development. These results provide new insights for the treatment of diseases relating to placental and embryonic development.