In female mammals, one X chromosome is transcriptionally inactivated (XCI), leading to dosage compensation between sexes, fundamental for embryo viability. A previous study using single-cell RNA-sequencing (scRNA-seq) data proposed that female human preimplantation embryos achieve dosage compensation by downregulating both Xs, a phenomenon named dampening of X expression. Using a novel pipeline on those data, we identified a decrease in the proportion of biallelically expressed X-linked genes during development, consistent with XCI. Moreover, we show that while the expression sum of biallelically expressed X-linked genes decreases with embryonic development, their median expression remains constant, rejecting the hypothesis of X dampening. In addition, analyses of a different dataset of scRNA-seq suggest the appearance of X-linked monoallelic expression by the late blastocyst stage in females, another hallmark of initiation of XCI. Finally, we addressed the issue of dosage compensation between the single active X and autosomes in males and females for the first time during human preimplantation development, showing emergence of X to autosome dosage compensation by the upregulation of the active X chromosome in both male and female embryonic stem cells. Our results show compelling evidence of an early process of X chromosome inactivation during human preimplantation development.