Grande M, et al. - This study is carried out to evaluate the distribution of the parental origin of the retained X chromosome in monosomy X, either in miscarriages or in ongoing pregnancies. The outcome suggests that the retained X chromosome in individuals with monosomy X should theoretically be maternally derived in 2/3 of the cases. This outcome proposes a preferential early miscarriage in pregnancies with a retained paternally derived X chromosome. This may clarify the observation that 75-90% of individuals with monosomy X retain the maternal X chromosome.