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Genome amplification and cellular senescence are commonly associated with pathological processes. While physiological roles for polyploidization and senescence have been described in mouse development, controversy exists over their significance in humans. Here, we describe tetraploidization and senescence as phenomena of normal human placenta development. During pregnancy, placental extravillous trophoblasts (EVTs) invade the pregnant endometrium, termed decidua, to establish an adapted microenvironment required for the developing embryo. This process is critically dependent on continuous cell proliferation and differentiation, which is thought to follow the classical model of cell cycle arrest prior to terminal differentiation. Strikingly, flow cytometry and DNAseq revealed that EVT formation is accompanied with a genome-wide polyploidization, independent of mitotic cycles. DNA replication in these cells was analysed by a fluorescent cell-cycle indicator reporter system, cell cycle marker expression and EdU incorporation. Upon invasion into the decidua, EVTs widely lose their replicative potential and enter a senescent state characterized by high senescence-associated (SA) β-galactosidase activity, induction of a SA secretory phenotype as well as typical metabolic alterations. Furthermore, we show that the shift from endocycle-dependent genome amplification to growth arrest is disturbed in androgenic complete hydatidiform moles (CHM), a hyperplastic pregnancy disorder associated with increased risk of developing choriocarinoma. Senescence is decreased in CHM-EVTs, accompanied by exacerbated endoreduplication and hyperploidy. We propose induction of cellular senescence as a ploidy-limiting mechanism during normal human placentation and unravel a link between excessive polyploidization and reduced senescence in CHM., Author summary In tissues, cellular differentiation is normally associated with cell cycle arrest. However, in some cases differentiating cells continue their cyclic activity in the absence of cell division. This event, referred to as endoreduplication, leads to polyploidy, which means an increased number of chromosome sets per cell and has been demonstrated in rodent placental trophoblast giant cells (TGCs). However, it is unknown whether human placental trophoblasts also endoreduplicate their genome. To study this, we focused on extravillous trophoblasts (EVTs), a specific trophoblastic subtype that invades the uterus during pregnancy in order to control blood supply and nutrient transfer to the growing embryo. We show that initiation of EVT differentiation is characterized by induced endoreduplication leading to genomic tetraploidization. Different to TGCs, EVTs duplicate their genome in an even manner. Upon invasion into the uterus, EVTs stop their endoreduplicative cycle and undergo cellular senescence. We further show that EVTs of hyperplastic complete hydatidiform mole (CHM) placentas, a genetic pregnancy disorder, are characterized by exacerbated endoreduplication, a greater DNA content and reduced signs for senescence. In summary, we propose senescence as a ploidy limiting factor during placental development and describe its suppression in hyperploid CHM-EVTs.