OBJECTIVES: To investigate the hormonal control and the possible consequences of uterine contractions (UC) on IVF-ET outcome. MATERIALS AND METHODS: We studied prospectively 220 controlled ovarian hyperstimulation (COH) cycles for IVF-ET. Just before ET, women underwent 5-minute digital recordings of the uterus using US image analysis software for UC assessment. Plasma progesterone (P) and estradiol were measured. Four groups were defined according to UC frequency: < or = 3.0 (n = 53), 3.1 to 4.0 (n = 50), 4.1 to 5.0 (n = 43), and > 5.0 (n = 74) UC/minute, respectively. RESULTS: Patients, COH and embryology characteristics were comparable in all groups. Notwithstanding estradiol levels were not associated with UC characteristics, plasma P and UC frequency were negatively correlated (r = -0.34, P < 0.001). A stepwise decrease in clinical and ongoing pregnancy as well as implantation rates occurred from the lowest to the highest UC frequency groups (53%, 36%, 21%; 46%, 32%, 20%; 23%, 19%, 10%; and 14%, 11%, 4%; P < 0.001). Direction of UC did not affect ET outcome. CONCLUSIONS: The negative correlation between UC frequency and P levels supports the utero-relaxing properties of P. High frequency UC on the day of ET hinder IVF-ET outcome, possibly by expelling embryos out of the uterine cavity.