Quality of embryos on day 7 after medium refreshment on day 6: a prospective trial

STUDY QUESTION: Are embryos that fail to meet biopsy or freezing criteria on day 6 (D6) more likely to meet these criteria on day 7 (D7) if cultured in fresh medium from D6 to D7? SUMMARY ANSWER: Refreshment of medium on D6 did not increase the proportion of usable embryos on D7, with an adverse effect for women >= 40 years old. WHAT IS KNOWN ALREADY: Embryo development in continuous single-step medium, from fertilization to the blastocyst stage, is equivalent to that using a sequential media protocol. However, there remains a theoretical benefit of refreshing the culture environment by transitioning slowly developing D6 embryos to a fresh medium droplet of the same composition, with a renewed source of nutrients and a milieu free of metabolic toxins. STUDY DESIGN, SIZE, DURATION: This was a prospective trial of culture media exposure in which embryos were randomized on D6 to remain in the same culture medium from D3 to D7 (continuous, n = 620) or be moved to fresh medium (fresh, n = 603) on D6, with re-evaluation on D7. Data were collected from IVF cycles, with or without ICSI, between 29 March 2019 and 17 February 2020. PARTICIPANTS/MATERIALS, SETTING, METHODS: Embryos from 298 women, aged 18-44 years, from cycles with or without preimplantation genetic testing (PGT) that did not meet criteria for biopsy and/or freeze on D6 were included in the study. Embryos were only included if there was a minimum of two embryos meeting the inclusion criteria in any cohort. Only the first cycle undertaken by each woman in the study period from which embryos were randomized was included. MAIN RESULTS AND THE ROLE OF CHANCE: A total of 1254 embryos were randomized from 312 cycles (209 non-PGT and 103 PGT) including 200 women undergoing IVF without PGT and 98 women who underwent PGT. The proportion of usable blastocysts on D7 did not differ between groups: 10.1% (61/603) in fresh versus 9.7% (60/620) in continuous medium (relative risk (RR) 1.05, 95% CI 0.74-1.47)). Embryos from women >= 40 years old had a significantly decreased likelihood of achieving a usable blastocyst on D7 after culture in fresh versus continuous medium: 3.5% versus 12.2%; RR 0.29, 95% CI 0.08-0.98. In total, 9.9% of embryos otherwise discarded on D6 met the criteria for biopsy and/or freeze on D7. LIMITATIONS, REASONS FOR CAUTION: Future work investigating implantation, clinical pregnancy and miscarriage rates with D7 embryos is still needed. WIDER IMPLICATIONS OF THE FINDINGS: Refreshment of medium on D6 did not increase the proportion of usable embryos on D7 overall. Younger women were more likely to develop D7 embryos after refreshment of medium on D6, while an adverse effect was seen in women >= 40 years old. However, by extending the culture of embryos to D7, additional blastocysts become available for clinical use.

Automated summary

The refreshment of medium on day 6 did not increase the proportion of usable embryos on day 7 overall, with an adverse effect seen in women aged 40 and above. Extending the culture of embryos to day 7 resulted in additional blastocysts available for clinical use.