RESUMO
Sex ratio in first trimester of pregnancy is skewed due to preferential elimination of female embryos. It could be resulted from aberrant X-chromosome inactivation. X-chromosome inactivation was analyzed in extraembryonic tissues of miscarriages and induced abortions with 46, XX karyotype. In chorion cytotrophoblast of both miscarriages and induced abortions observed either random or skewed X-chromosome inactivation. In extraembryonic mesoderm of the control group, random inactivation was observed, whereas 15% of miscarriages had skewed X-chromosome inactivation. The highest frequency of skewed inactivation of one of the parental homologues was observed in the groups of blighted ovum pregnancy and embryos from women with recurrent pregnancy loss. It was suggested that in these cases compartmentalization of cells in the blastocyst probably leads to predominance of cell with mutant active X-chromosome among the cells of inner cell mass carrying the aberrations that are incompatible with normal embryonic development.