Pregnancy and implantation rates in normal replacement versus stimulated cycles

The relative importance that endometrial development and embryo quality have on implantation rates achieved with assisted reproductive technology (ART) is the subject of controversy. Ovarian stimulation has been repeatedly mentioned as having a detrimental effect on endometrial receptivity (Paulson et al., 1990, Fertil. Steril., 53, 870-874). We compared pregnancy and implantation rates achieved with ART during stimulated cycles and hormonal replacement cycles, in patients matched for the following criteria: age < 35 years for the patient donating oocytes; transfer of at least two good quality embryos/oocytes and good quality transfer. All transfer cases performed during hormonal replacement cycles were done with donated oocytes. Comparison of results between techniques was not attempted due to potential differences in populations. The pregnancy and implantation rates achieved with each technique during stimulated and hormonal replacement cycles were not statistically different. In contradiction to previous results, our data suggest that differences in uterine receptivity between stimulated and hormonal replacement cycles in the age group studied are not of critical importance in embryo implantation. Good embryo quality appears to be the dominant factor in determining the success of ART.