What is the best strategy for slowly developing blastocysts?

Day 5 fresh blastocyst transfers results in higher clinical pregnancy and live birth rates than day 6 fresh blastocyst transfer. This study aimed to identify the strategy to adopt with slowly developing blastocysts. Should not fully expanded blastocyst on day 5 be transferred on day 5, or when expanded on day 6, or be frozen? 1093 single blastocyst transfer cycles performed between January 2016 and December 2018 were divided in 4 groups: day 5 fresh transfers of full or expanded blastocyst (≥B3), day 5 fresh transfers of slowly developing blastocysts (B1 or B2), day 6 fresh transfers of expanded blastocysts (≥B4), day 6 frozen-thawed single blastocyst transfer cycles. Clinical pregnancy rate and live birth rate were significantly higher with fresh expanded blastocyst transfer on day 5 than in any other group. No statistical difference could be found between the other 3 groups. Slowly developing day 5 blastocysts have poorer implantation potential than expanded day 5 blastocysts but can be fresh transferred on day 5 rather than being cultured until day 6 for transfer or freezing when no expanded blastocyst is available on day 5.