OBJECTIVE: To compare changes in uterine contraction (UC) frequency occurring after ovulation in the menstrual cycle and in IVF and to clarify UC anomalies encountered in IVF. DESIGN: Prospective crossover trial. SETTING: University infertility clinic. PATIENT(S): Six women seeking IVF for male factor infertility having regular menstrual cycles were studied during the menstrual cycle and IVF. INTERVENTION(S): Identification of LH surge in the menstrual cycle. Measurement of plasma E(2) and P levels and assessment of UC frequency by direct ultrasound visualization on the day of LH surge in the menstrual cycle and the day of hCG administration in IVF and every 2 days thereafter for 6 days. MAIN OUTCOME MEASURE(S): Uterine contraction (UC) frequency and plasma E(2) and P levels. RESULT(S): UC frequency was similar on the day of LH surge (5/minute) and hCG administration (5.3/minute). Establishment of uteroquiescence was more prompt in the menstrual cycle than in IVF, with lower UC frequency 4 days after LH surge compared with 4 days after hCG, while E(2) and P levels were higher in IVF. Six days after LH surge/hCG administration, UC frequency was low in the menstrual cycle and in IVF. CONCLUSION(S): High UC frequency in IVF at the time of ET results from delayed establishment of uteroquiescence after ovulation in IVF as compared with the menstrual cycle. In IVF, low UC frequency 6 days after hCG may contribute to the higher pregnancy rates observed with blastocyst transfers.