This study investigates the dynamic regulation of DNA methylation during early mammalian embryogenesis. The authors generated genome-scale DNA methylation maps in mouse gametes and post-implantation embryos, revealing that oocytes exhibit global hypomethylation, particularly at specific families of long interspersed element-1 and long terminal repeat retroelements. Surprisingly, oocytes contribute a unique set of Differentially Methylated Regions (DMRs), including many CpG Island promoter regions, which are maintained in the early embryo but lost upon specification and absent from somatic cells. In contrast, sperm-contributed DMRs are largely intergenic and resolve to hypermethylation after the blastocyst stage. These findings provide a comprehensive, base-resolution timeline of DNA methylation in the pre-specified embryo, highlighting the dynamic nature of this epigenetic modification before it returns to the canonical somatic pattern.This study investigates the dynamic regulation of DNA methylation during early mammalian embryogenesis. The authors generated genome-scale DNA methylation maps in mouse gametes and post-implantation embryos, revealing that oocytes exhibit global hypomethylation, particularly at specific families of long interspersed element-1 and long terminal repeat retroelements. Surprisingly, oocytes contribute a unique set of Differentially Methylated Regions (DMRs), including many CpG Island promoter regions, which are maintained in the early embryo but lost upon specification and absent from somatic cells. In contrast, sperm-contributed DMRs are largely intergenic and resolve to hypermethylation after the blastocyst stage. These findings provide a comprehensive, base-resolution timeline of DNA methylation in the pre-specified embryo, highlighting the dynamic nature of this epigenetic modification before it returns to the canonical somatic pattern.