De Munck N, et al. - Researchers investigated the benefit of reducing the oxygen tension from 5% to 2% during extended culture from Day 3 onwards for human blastocyst development in vitro. In two prospective studies performed between December 2016 and September 2017, they randomized sibling embryos on Day 3 to either 5% O₂ (control) or 2% O₂ (study) for extended culture. In the 2% study groups, they performed the overnight pre-equilibration of blastocyst media in either 2% O₂ (study 1, 99 cycles) or 5% O₂ (study 2, 126 cycles). The latter provides a gradual transition from 5 to 2% O₂ environment for the study arm. In study 1, randomization was done of 811 embryos on Day 3: 405 to the 2% O₂ and 406 to the 5% O₂ condition. In study 2, randomization was done of 1144 embryos: 572 in each arm. Results demonstrate no benefit in lowering the oxygen tension from 5 to 2% from Day 3 onwards during extended human embryo culture.