OBJECTIVE: To study the association between the number of blastocysts available and pregnancy outcomes in first fresh autologous single blastocyst transfer cycles. DESIGN: Retrospective cohort study. SETTING: Not applicable. PATIENT(S): Patients from the Society for Assisted Reproductive Technology reporting fertility clinics (n=16,666). INTERVENTIONS(S): None. MAIN OUTCOME MEASURE(S): Primary outcomes were clinical pregnancy (CP), live birth (LB), and miscarriage rates. Logistic regression was used to investigate the association between the number of blastocysts and each outcome. RESULT(S): When comparing fresh single blastocyst transfer rates, the odds of a positive pregnancy outcome (CP) increased significantly with each additional supernumerary blastocyst up to five and declined by 2% for every additional blastocyst after five. Similarly, the odds of an LB was 17% higher for each additional blastocyst up to five and declined by 2% for every additional blastocyst after five. There was no significant association between blastocyst number and miscarriage rate. CONCLUSION(S): Odds of positive pregnancy outcomes (CP, LB) increased significantly with every additional blastocyst up to five, but declined after that, in first fresh autologous cycles with single-blastocyst transfer. The decline after five may be explained by a detrimental effect on endometrial receptivity in patients with a large number of oocytes or inadequate selection of the best embryo for transfer based on morphology alone.