Follicular and estradiol parameters that improve success with natural cycle in vitro fertilization.

OBJECTIVE: To describe clinical thresholds for follicle size and estradiol levels to optimize success with natural cycle in vitro fertilization (NCIVF).

STUDY DESIGN: Descriptive cohort of candidates for stimulated IVF, < 43 years old, with regular menstrual cycles, regardless of ovarian reserve or fertility treatment history. Patients underwent NCIVF, defined as oocyte retrieval, fertilization and embryo transfer after human chorionic gonadotropin (hCG) trigger without luteinizing hormone (LH) suppression or ovarian stimulation medications.

RESULTS: A total of 422 patients underwent 821 NCIVF cycles. Clinical pregnancy rates per cycle start, retrieval, and transfer were 13%, 17%, and 32%, respectively, for all patients and 19%, 25%, and 49% for patients < 30 years old. The threshold estradiol level on day of hCG was 101 pg/mL; below that level no clinical pregnancies occurred. Likewise, a mean follicular diameter > 15 mm was the optimal threshold for hCG trigger. Anti-Müllerian hormone and follicle-stimulating hormone levels did not predict success in NCIVF, and no statistical difference in clinical pregnancy rates between day 3 or day 5 embryo transfer was observed.

CONCLUSION: NCIVF is an effective therapy for infertile patients regardless of their ovarian reserve. Cycle cancellation due to a premature LH surge can be reduced, without sacrificing success, by triggering smaller follicles above a threshold level of estradiol.