Clinical pregnancy and live birth increase significantly with every additional blastocyst up to five and decline after that: an analysis of 16,666 first fresh single-blastocyst transfers from the Society for Assisted Reproductive Technology registry.

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.