Background: Bi-directional communication between the follicle and oocyte is necessary to regulate follicle and oocyte development. Currently, it is not practical to monitor the serial growth of individual follicles during assisted reproduction. The ovarian stimulation phase length (SPL) is an indirect measure of mean follicular growth rate. The objective of this study was to test the hypothesis that a short or long SPL would be associated with suboptimal outcomes in women undergoing in vitro fertilization (IVF).Materials and Methods: A retrospective cohort study was conducted in 140 women who underwent IVF. Follicle development was monitored every 2-3 days during ovarian stimulation using transvaginal ultrasonography. Once>3 follicles reached ³ 17 mm, human chorionic gonadotropin (hCG) was administered. Oocyte retrieval was performed approximately 35 hours after hCG.Oocytes underwent IVF on the day of collection and were evaluated daily thereafter. Embryos were transferred on days 3 or 5, depending on the number and quality of embryos available. Associations between SPL, age, follicle, oocyte, embryo and pregnancy outcomes were evaluated (SPSS version 17.0; SPSS Inc., Chicago, IL, USA).Results: A SPL of 11 days was associated with an optimal number of follicles that developed to ≥ 6 mm, ≥ 10 mm and ³ 15 mm; serum estradiol concentrations; and number of oocytes collected (p<0.05). Gradual reductions in the number of developing follicles, serum estradiol concentrations and number of oocytes collected occurred with SPL less than or greater than 11 days (p<0.05). The SPL did not influence endometrial, embryo or pregnancy outcomes (p>0.05). Associations between SPL and outcomes were not influenced by age (p>0.05).Conclusion: The ovarian SPL can be used to predict the number of follicles that develop, oocytes collected and serum estradiol concentrations, but not embryo or pregnancy outcomes.