Clinical pregnancy rate following frozen embryo transfer is higher with blastocysts vitrified on day 5 than on day 6

Purpose: The aim of this study was to compare the pregnancy rates between good quality blastocysts vitrified on day 6 versus blastocysts vitrified on day 5 after fertilization. Methods: This is a retrospective cohort study of 791 freeze-thaw cycles of blastocysts vitrified either on day 5 or on day 6 and transferred between January 2012 and October 2015. Five hundred and thirty-seven cycles included blastocysts vitrified on day 5, and 254 cycles included blastocysts vitrified on day 6. Results: The age of the patients and the proportion of embryos that survived the thawing process were comparable between the two groups. More good quality embryos were transferred in the group in which blastocysts were vitrified on day 6 (1.2 vs. 1.3, p = 0.005), but the clinical pregnancy rate (44 vs. 33 %, p = 0.002) and the ongoing pregnancy rate (41 vs. 28 %, p < 0.001) were higher in the group in which blastocysts were vitrified on day 5. Multivariate regression analysis adjusting for patient’s age, number of good quality embryos transferred (≥3BB), and treatment protocol demonstrated that the day 6 vitrified group had a significantly lower clinical pregnancy rate compared to the day 5 vitrified group (OR 0.54, 95 % CI 0.38–0.76). Conclusions: The clinical pregnancy rate following frozen embryo transfer is significantly lower with blastocysts vitrified on day 6 compared to blastocysts vitrified on day 5.