Transvaginal ultrasonography with colour blood flow imaging and analysis of impedance to uterine arterial blood flow has been used to provide an index of uterine receptivity for implantation after IVF/embryo transfer. A mean uterine arterial pulsatility index (PI) > 3.0 at the time of embryo transfer predicted 35% of failures to become pregnant. Cryopreserving embryos in non-receptive cycles and transferring them in receptive cycles would be expected to improve pregnancy rates. Earlier decisions regarding embryo cryopreservation can be made if receptive cycles can be predicted at the time of oocyte retrieval rather than at embryo transfer. To assess differences in uterine artery impedance, PI were measured serially in 107 women on both the day of oocyte retrieval and the day of embryo transfer. Mean PI on the day of oocyte retrieval was 2.52 +/- 0.59, and on the day of embryo transfer was 2.78 +/- 0.45. No significant difference was observed when PI determined on the day of oocyte retrieval were compared with PI on the day of embryo transfer. These data suggest that the PI measurement done on the day of oocyte retrieval could substitute for the measurement done on the day of embryo transfer. This would allow prediction of non-receptive endometria earlier in the cycle. Further studies are needed to evaluate whether cryopreservation of embryos and transfer when the uterus is more receptive will increase the implantation rates.