STUDY DESIGN, SIZE, DURATION: A secondary analysis of a previous randomized controlled trial assessing clinical pregnancy outcomes was carried out. This analysis included 1125 consecutive oocyte donation cycles utilizing ICSI or IVF and Day 3 embryo transfers between November 2009 and April 2012. The whole cohort of donated oocytes from patients who agreed to participate in the study were randomly allocated (1:1 ratio) to a reduced O2 tension group (6% O2) or an air-exposed group (20% O2) based on a computergenerated randomization list. Fresh and vitrified oocytes were used for oocyte donation. Only those pregnancies with a live birth at or beyond 24 weeks of gestation were included. PARTICIPANTS/MATERIALS, SETTING, METHODS: Day 3 embryos were cultured in an atmosphere of 5.5% CO2, 6% O2, 88.5% N2 versus a dual gas system in air. MAIN RESULTS AND THE ROLE OF CHANCE: From the eligible 1125 cycles, 564 were allocated to the 6% O2 group and 561 cycles to the 20% O2 group. However, 50 and 62 cycles did not reach embryo transfer in the 6% and 20% O2 groups, respectively. No differences were found between 6% O2 and atmospheric O2 tension in the number of livebirths per embryo transfer (mean § SD, 0.5 § 0.7 versus 0.5 § 0.7), pregnancy complications or neonatal outcomes. Both groups (6% and atmospheric O2) had similar single and twin delivery rates (40.8% versus 38.1% and 10.7% versus 12.3%, respectively). Preterm delivery rates and very preterm delivery rates (10.80% versus 13.24% and 1.25% versus 2.94%, respectively), birthweight (3229 § 561 g versus 3154 § 731 g), low birthweight (2.92% versus 2.45%), birth height (50.18 § 2.41 cm versus 49.7 § 3.59 cm), head circumference (34.16 § 1.87 cm versus 33.09 § 1.85 cm) and 1 min Apgar scores (8.96 § 0.87 versus 8.89 § 0.96) were also similar between 6% and atmospheric O2 groups, respectively. LIMITATIONS, REASONS FOR CAUTION: The number of liveborns finally analyzed is still small and not all obstetric and neonatal variables could be evaluated. Furthermore, a small proportion of the obstetric and neonatal data was obtained through a questionnaire VC The Author(s) 2020. Published by Oxford University Press on behalf of European Society of Human Reproduction and Embryology. All rights reserved. For permissions, please email: journals.permissions@oup.com Human Reproduction, Vol.0, No.0, pp. 1-14, 2020 doi:10.1093/humrep/deaa152 One reason for the lack of effect of oxygen concentration on pregnancy outcome could be the absence of trophectoderm cells at cleavage stage, which may make Day 3 embryos less susceptible to hypoxic conditions. WIDER IMPLICATIONS OF THE FINDINGS: Nowadays many IVF laboratories use a more physiological oxygen concentration for embryo culture. However, the benefits of using low oxygen concentration on both laboratory and clinical outcomes during embryo culture are still under debate. Furthermore, long-term studies investigating the effect of using atmospheric O2 are also needed. Gathering these type of clinical data is indeed, quite relevant from the safety perspective. The present data show that, at least in egg donation cycles undergoing Day 3 embryo transfers, culturing embryos under atmospheric oxygen concentration seems not to affect perinatal outcomes