Discussion
Principal findings
For women aged 36 years and older with at least two blastocysts, there was no significant difference in the odds of cumulative live birth following single- versus double- blastocyst transfer accounting for female age and blastocysts quality. Overall, time needed to achieve live birth did not decrease with transfer of two blastocysts at one time, but the risk of multiple livebirths and adverse perinatal outcomes did increase.