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Objectives: To compare the effects of atosiban (oxytocin antagonist) on uterine peristalsis and pregnancy outcomes in the frozen embryo transfer (FET) cycle.Setting: Srinagarind Hospital, a university hospital, Khon Kaen, Thailand.Design: A randomized, double-blinded, controlled trial.Methods: Fifty infertile women were randomized into the atosiban (n = 25) and placebo group (n = 25). Women in the study group received intravenous atosiban 6.75 mg, 30 min before embryo transfer, and continued infusion at 18 mg/h for 1 h. The dose was reduced to 6 mg/h for another 2 h. Saline solution was applied in the placebo group. The uterine peristalsis frequency was measured by transvaginal ultrasound 30 min before and 3 h after the embryo transfer.Results: The respective mean baseline uterine peristalsis frequency (time) in the atosiban and placebo group was 10.3 ± 2.4 and 9.2 ± 3.4. The respective duration of uterine peristalsis in the atosiban and placebo group after receiving the intervention was reduced to 7.9 ± 2.1 and 6.9 ± 2.7. The implantation rate and clinical pregnancy rate were not statistically significant different between atosiban group and placebo group (37.5% versus 31.0%, RR 1.21, 95%CI: 0.60-2.44 and 44% versus 36%, RR 1.22, 95%CI: 0.62-2.42, respectively). Subgroup analysis indicated that the clinical pregnancy rate in those >35 years of age was not significantly different between both groups (31.6% and 18.8 %, RR 1.68, 95%CI: 0.50-5.68).Conclusion: Adding atosiban in FET did not reduce uterine peristalsis but may benefit the advanced age group. [ABSTRACT FROM AUTHOR]