Skewed X inactivation and recurrent spontaneous abortion.

Recent studies show that women experiencing recurrent spontaneous abortion exhibit nonrandom X-chromosome inactivation (XCI) more often than in controls. This suggests that genetic factors may be important in explaining the losses in this subset of women. Nonetheless there are a number of possible explanations for this finding and the underlying causes may be heterogeneous. One hypothesis commonly cited is that a mutation on the X chromosome results in both preferential inactivation of the mutated X as well as lethality of male embryos inheriting this mutated X. However, this hypothesis does not explain the increase in chromosome abnormalities observed in the karyotyped losses from women with recurrent pregnancy loss and skewed XCI. This finding leads us to suggest that the mechanism involved may be associated with a reduction in number of ovarian follicles, either due to X mutations affecting oocyte atresia or a restriction in precursor pool size during development.