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Haemoglobin expression is not restricted to erythroid cells. We investigated the gene expression of the haemoglobin subunits haemoglobin, alpha adult chain 1 (Hba-a1) and haemoglobin, beta (Hbb), 2,3-bisphosphoglyceratemutase (Bpgm) and the oxygen-regulated genes BCL2/adenovirus E1B interacting protein 3 (Bnip3), solute carrier family2 (facilitated glucose transporter), member 1 (Slc2a1) and N-myc downstream regulated gene 1 (Ndrg1) in the murine preimplantation embryo, comparing in vivo to in vitro gene expression. Relatively high levels of Hba-a1 and Hbb were expressed in vivo from the 2-cell to blastocyst stage; in contrast, little or no expression occurred in vitro. We hypothesised that the presence of haemoglobin in vivo creates a low oxygen environment to induce oxygen-regulated gene expression,supported by high expression of Slc2a1 and Ndrg1 in in vivo relative to in vitro embryos. In addition, analysis of an in vitro derived human embryo gene expression public dataset revealed low expression of haemoglobin subunit alpha (HBA) and HBB, and high expression of BPGM. To explore whether there was a developmental stage-specific effect of haemoglobin,we added exogenous haemoglobin either up to the 4-cell stage or throughout development to the blastocyst stage, but observed no difference in blastocyst rate or the inner cell mass to trophectoderm cell ratio. We conclude that haemoglobin in the in vivo preimplantation embryo raises an interesting premise of potential mechanisms for oxygen regulation, which may influence oxygen-regulated gene expression. Refereed/Peer-reviewed