OBJECTIVE: To investigate whether uterine contractility at the time of embryo transfer (ET) can be reduced by early onset of luteal support with progesterone administered vaginally. DESIGN: Prospective analysis. SETTING: Assisted reproduction unit. PATIENT(S): Eighty-four women undergoing 84 GnRH-a and FSH/hCG cycles for IVF-ET were studied. INTERVENTION(S): Vaginal progesterone was randomly started on the day of oocyte retrieval (group A, n = 43) or on the evening of ET (group B, n = 41). On the day of hCG administration and just before ET, 2-minute sagittal uterine scans were obtained by ultrasound and digitized with an image analysis system for assessing uterine contraction frequency. MAIN OUTCOME MEASURE(S): Uterine contraction frequency. RESULT(S): Whereas uterine contraction frequency was similar in both groups on the day of hCG (4.6 +/- 0.3 and 4.5 +/- 0.3 contractions per minute, respectively), only women in group A showed decreased uterine contraction frequency on the day of ET (2.8 +/- 0.2 vs. 4.2 +/- 0.3 contractions per minute). CONCLUSION(S): Vaginal progesterone administration starting on the day of oocyte retrieval induced a decrease in uterine contraction frequency on the day of ET as compared with preovulatory values. Uterine relaxation before ET is likely to improve IVF-ET outcome by avoiding the displacement of embryos from the uterine cavity.