Day 4 embryo selection is equal to Day 5 using a new embryo scoring system validated in single embryo transfers

BACKGROUND: Single embryo transfers (SETs) require the most viable embryo from a cohort to be selected. Day 4 embryos may provide a selection advantage similar to blastocyst transfer as embryos are replaced post-embryonic genome activation and into the uterus where they would normally reside. However, there is currently no adequate morphological system to assess Day 4 embryos. Therefore, we developed an assessment system for Day 4 embryos and retrospectively assessed Day 4 SET success rates compared with Day 5 SETs. METHODS: Embryos (n = 996 after IVF or ICSI) were assessed on Day 4 of development, prior to implementing Day 4 embryo transfers, to obtain a descriptive range of embryo morphologies observed at this time point. A morphological scoring system was developed from this. All patients having an SET after extended culture (Day 4, n = 124 or Day 5, n = 193), at Repromed, Adelaide, Australia, during June 2006-January 2007 were analysed for pregnancy outcome. Ongoing pregnancy was determined by fetal cardiac activity at 6-8 weeks after oocyte collection. RESULTS: Day 4 and 5 SETs resulted in similar ongoing pregnancy rates of 38.7% and 32.1%, respectively. CONCLUSIONS: A Day 4 scoring system was successfully developed and implemented. Day 4 SETs were found to be a viable option or alternative to Day 5 SETs with no difference in pregnancy rates.