Purpose <p>The purpose of this study was to identify genes in the placenta of monosomy X embryos whose methylation abnormalities may be associated with embryonic death.</p> Methods <p>Methylation levels were assessed via reduced representation bisulfite sequencing of the chorionic villi of embryos from 8 spontaneous abortions during the first trimester of pregnancy with the 45,X karyotype and 7 embryos from medical abortions with the 46,XX and 46,XY karyotypes. The methylation levels of several identified differentially methylated genes were analyzed in 22 embryos from spontaneous abortions with monosomy X compared with 11 embryos from medical abortions using targeted bisulfite massive parallel sequencing.</p> Results <p>Compared with embryos with 46,XX and 46,XY karyotypes, respectively, differentially methylated CpG sites in embryos with monosomy X were located in 831 and 254 differentially methylated genes (DMGs). However, only 74 DMGs were unique for 45,X embryos after subtraction of the DMG of spontaneously aborted embryos with a normal karyotype (<i>n</i> = 4). Compared with embryos of both sexes, 48 genes in embryos with monosomy X were differentially methylated, 21 of which are important in normal placental and embryonic development and whose dysregulation is linked to preeclampsia and embryonic death. Targeted analysis confirmed that the <i>ALCAM</i> gene is hypermethylated and that the <i>BDH1</i> gene is hypomethylated in embryos with monosomy X.</p> Conclusion <p>Aberrant methylation of genes involved in placentation, proliferation, and cell differentiation has been detected in spontaneously aborted embryos with monosomy X. These disorders may be associated with the high lethality of embryos with monosomy X.</p>