Objective: We tested the hypothesis that preterm delivery in women with twins or polyhydramnios is associated with enhanced expression and activity of cyclooxygenase type 2 in the amnion. Study Design: We obtained amniotic tissue samples from women undergoing preterm delivery complicated by either twins or polyhydramnios and also from women undergoing preterm cesarean delivery before or after labor. We measured amniotic prostaglandin E₂ content and determined cyclooxygenase type 1 and type 2 expressions. We inhibited cyclooxygenase type 1 and type 2 activities with selective inhibitors. Results: Preterm delivery attributed to twins or polyhydramnios was associated with higher prostaglandin E₂ production and enhanced amniotic expression of cyclooxygenase type 2. In contrast, cyclooxygenase type 1 expression was unchanged. Incubation of amniotic samples in vitro with either indomethacin or the selective cyclooxygenase type 2 inhibitor SC-236, but not with the cyclooxygenase type 1 inhibitor SC-560, effectively reduced prostaglandin E₂ production. Conclusion: Preterm delivery related to multiple gestation or polyhydramnios was associated with enhanced amniotic expression and activity of cyclooxygenase type 2. (Am J Obstet Gynecol 2001;184:117-22.)