Objective: To assess the usefulness of the GnRH antagonist cetrorelix to prevent LH surge and to avoid intrauterine insemination at weekends when a gynecologist on duty is not available and the ultrasound scan on Friday showed >1 and < 3 follicles ≥ 17 mm in diameter. Design: Open- label, randomized, prospective study. Setting: Reproductive medicine unit in an acute care teaching hospital in Barcelona, Spain. Patient(s): Infertile patients undergoing controlled ovarian hyperstimulation (COH) and IUI. Intervention(s): Treatment with recombinant FSH was started on day 3. In women assigned to the control group (n = 32), recombinant FSH was continued up to the day of hCG administration. In patients assigned to the GnRH antagonist group (n = 35), half of the dose of recombinant FSH was given for 2 more days in addition to cetrorelix (0.25 mg SC) until the day of hCG administration. Main Outcome Measure(s): Recombinant FSH doses, E2 level on the day of hCG administration, number and diameter of follicles, endometrial thickness, and number of pregnancies. Result(s): Only a case of premature ovulation occurred in the cetrorelix group. There were no significant differences between the study groups in the total mean number of follicles, follicles >10 mm and < 17 mm, and follicles ≥ 17 mm. The mean concentration of E2 on the day of hCG administration and the endometrial thickness were significantly higher in the cetrorelix group. Eleven pregnancies were achieved, 7 (20% ) in the cetrorelix group (4 singleton, 3 twins) and 4 (12.5% ) in controls (4 singleton). No case of ovarian hyperstimulation syndrome (OHSS) occurred. Conclusion(s): The use of cetrorelix to avoid IUI at weekends when the ultrasound scan on Friday shows > 1 and < 3 follicles ≥ 17 mm is a useful alternative for medical centers in which a gynecologist on call is not available. Objective: To assess the usefulness of the GnRH antagonist cetrorelix to prevent LH surge and avoid avoid intrauterine insemination at weekends when a gynecologist on duty is not available and the ultrasound scan on Friday showed> 1 and <3 follicles ≥ 17 mm in diameter. Design Setting: Reproductive medicine unit in an acute care teaching hospital in Barcelona, ​​Spain. Patient (s): Infertile patients undergoing controlled ovarian hyperstimulation (COH) and IUI. Intervention (s): Treatment with Infected assigned to the control group (n = 32), recombinant FSH was continued up to the day of hCG administration. In patients assigned to the GnRH antagonist group (n = 35), half of the The dose of recombinant FSH was given for 2 more days in addition to cetrorelix (0.25 mg SC) until the day of hCG administration. Main Outcome Measure (s): Recombinant FSH doses, E2 level on the day of hCG administration, number and diamet er of follicles, endometrial thickness, and number of pregnancies. Result (s): Only a case of premature ovulation occurred in the cetrorelix group. There were no significant differences between the study groups in the total mean number of follicles, follicles> 10 mm and <17 mm, and follicles ≥ 17 mm. The mean concentration of E2 on the day of hCG administration and the endometrial thickness were significantly higher in the cetrorelix group. Eleven pregnancies were achieved, 7 (20%) in the cetrorelix group (4 No case of ovarian hyperstimulation syndrome (OHSS) occurred. Conclusion (s): The use of cetrorelix to avoid IUI at weekends when the ultrasound scan on Friday shows > 1 and <3 follicles ≥ 17 mm is a useful alternative for medical centers in which a gynecologist on call is not available.