Chromosomal abnormalities in human pre-implantation embryos, originating from either meiotic or mitotic errors, present a significant challenge in reproductive biology. Complete aneuploidy is primarily linked to errors during the resumption of meiosis in oocyte maturation, which increase with maternal age, while mosaic aneuploidies result from mitotic errors after fertilization. The biological causes of these abnormalities are increasingly becoming a topic of interest for research groups and clinical specialists. This review explores the intricate processes of meiotic and early mitotic divisions in embryos, shedding light on the mechanisms that lead to changes in chromosome number in daughter cells. Key factors in meiotic division include difficulties in spindle assembly without centrosomes, kinetochore (KT) orientation disturbances, and inefficient cell-cycle checkpoints. The weakening of cohesion molecules that bind chromosomes, exacerbated by maternal aging, further complicates chromosomal segregation. Mitotic errors in early development are influenced by defects in sperm centrosomes, KT misalignment, and the gradual depletion of maternal regulatory factors. Coupled with the inactive or partially active embryonic genome, this depletion increases the likelihood of chromosomal aberrations. While various theoretical mechanisms for these abnormalities exist, current data remain insufficient to determine their exact contributions. Continued research is essential to unravel these complex processes and improve outcomes in assisted reproductive technologies.