Pushing the Limits: Day 7 Embryo Culture

Abstract

Improvements in embryo culture systems, based on the dynamic metabolic requirements of the embryo have facilitated the extension of embryo culture to the blastocyst stage. In the last 20 years, there has been a shift to blastocyst culture and single blastocyst transfer around the world, especially in Australia and New Zealand. Notably, this practice has led to significantly increased implantation and pregnancy rates, by improving embryo selection and embryo-uterine environment synchrony. Interestingly, the time taken for a human embryo to develop into a blastocyst can vary. Blastocysts are typically selected for transfer, biopsy or vitrification on days 5 and 6 of embryo development, and embryos which do not reach a suitable developmental stage and/or quality are typically discarded. Over the last decade, multiple groups have challenged this ‘norm’. It has been determined that slow-growing embryos that only reach the blastocyst stage on day 7 have the ability to attain a good morphological grade, successively undergo biopsy and vitrification, and most importantly, can result in healthy live births following transfer. With the advent of time-lapse microscopy, we can now closely examine the timings of embryo development. Here, the development and incidence of day 7 blastocysts, metabolic requirements, rate of euploidy, reproductive potential and laboratory experience of extending embryo culture to day 7 are considered.