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Background: The environmental oxygen tension has been reported to impact the blastocyst quality and cell numbers in the inner cell mass (ICM) during human and murine embryogenesis. While the molecular mechanisms leading to increased ICM cell numbers and pluripotency gene expression under hypoxia have been deciphered, it remains unknown which regulatory pathways caused the underweight fetal body and overweight placenta after maternal exposure to hyperbaric oxygen (HBO). Results: The blastocysts from the HBO‐exposed pregnant mice revealed significantly increased signals of reactive oxygen species (ROS) and nuclear Nrf2 staining, decreased Nf2 and Oct4 expression, increased nuclear Tp53bp1 and active caspase‐3 staining, and ectopic nuclear signals of Cdx2, Yap, and the Notch1 intracellular domain (N1ICD) in the ICM. In the ICM of the HBO‐exposed blastocysts, both Nf2 cDNA microinjection and Nrf2 shRNA microinjection significantly decreased the ectopic nuclear expression of Cdx2, Tp53bp1, and Yap whereas increased Oct4 expression, while Nrf2 shRNA microinjection also significantly decreased Notch1 mRNA levels and nuclear expression of N1ICD and active caspase‐3. Conclusion: We show for the first time that maternal exposure to HBO at the preimplantation stage induces apoptosis and impairs ICM cell specification via upregulating Nrf2‐Notch1‐Cdx2 expression and downregulating Nf2‐Oct4 expression. Key Findings: Compared to the blastocysts derived from normoxia‐exposed female mice, the blastocysts from hyperbaric oxygen (HBO)‐exposed mothers exhibited significantly increased levels of reactive oxygen species (ROS) and nuclear Nrf2 staining, along with significantly decreased membranous Nf2 staining throughout the entire embryo in immunofluorescence analyses. Additionally, there was a significant decrease in Oct4 expression, increased nuclear staining of Tp53bp1 and active caspase‐3, and ectopic expression of Cdx2, Yap, and the Notch1 intracellular domain (N1ICD) specifically within the inner cell mass (ICM).Both microinjections of Nf2 overexpressor cDNA and Nrf2 shRNA into the pronuclear zygotes followed by oviduct transfer before daily HBO exposures significantly decreased the nuclear expression of Cdx2, Tp53bp1 and Yap whereas increased Oct4 expression in the ICM of the HBO‐exposed blastocysts.Microinjection of Nrf2 shRNA into the pronuclear zygotes before daily HBO exposures significantly decreased Notch1 mRNA levels and nuclear expression of ectopic N1ICD and active caspase‐3 in the ICM of the HBO‐exposed blastocysts.Microinjection of Nf2 overexpressor cDNA into the pronuclear zygotes before HBO exposure did not affect the levels of cell apoptosis, Notch1 mRNA and ectopic N1ICD expression in the ICM of the HBO‐exposed blastocysts. [ABSTRACT FROM AUTHOR]