Human aneuploidy, a condition where chromosomes are present in an abnormal number, is a significant cause of pregnancy loss and birth defects. It occurs in at least 10% of human pregnancies, with higher rates in older women. Aneuploidy primarily results from errors in meiosis, particularly in the oocyte. Recent studies have shown that the age-related increase in aneuploidy is due to a combination of factors, including the prolonged meiotic arrest in oocytes and various endogenous and exogenous influences. Aneuploidy is a major cause of miscarriage and congenital disorders, with trisomy 21 (Down syndrome) being the most well-known example. Studies of live births and miscarriages have shown that aneuploidy is more common in early pregnancies, and recent advances in preimplantation genetic diagnosis have provided new insights into the mechanisms of aneuploidy. These studies suggest that aneuploidy rates are higher in ART-derived pregnancies than previously thought, but recent methods like array-based analyses have provided more accurate estimates. Sex-specific differences in meiosis contribute to aneuploidy, with female meiosis being more error-prone. This is partly due to the prolonged meiotic arrest in oocytes and the complex interplay of factors affecting chromosome segregation. The loss of sister chromatid cohesion is a key mechanism contributing to aneuploidy, especially with age. Studies in mice and humans have shown that cohesin loss is a significant factor in age-related aneuploidy. Environmental factors such as exposure to endocrine disruptors like bisphenol A (BPA) can also influence aneuploidy. BPA exposure has been linked to meiotic errors and altered oocyte development. Additionally, ART procedures may increase aneuploidy rates, highlighting the need for further research into the mechanisms underlying aneuploidy and the development of strategies to reduce its occurrence. Understanding these factors is crucial for improving reproductive health and addressing the challenges of aneuploidy in human pregnancies.Human aneuploidy, a condition where chromosomes are present in an abnormal number, is a significant cause of pregnancy loss and birth defects. It occurs in at least 10% of human pregnancies, with higher rates in older women. Aneuploidy primarily results from errors in meiosis, particularly in the oocyte. Recent studies have shown that the age-related increase in aneuploidy is due to a combination of factors, including the prolonged meiotic arrest in oocytes and various endogenous and exogenous influences. Aneuploidy is a major cause of miscarriage and congenital disorders, with trisomy 21 (Down syndrome) being the most well-known example. Studies of live births and miscarriages have shown that aneuploidy is more common in early pregnancies, and recent advances in preimplantation genetic diagnosis have provided new insights into the mechanisms of aneuploidy. These studies suggest that aneuploidy rates are higher in ART-derived pregnancies than previously thought, but recent methods like array-based analyses have provided more accurate estimates. Sex-specific differences in meiosis contribute to aneuploidy, with female meiosis being more error-prone. This is partly due to the prolonged meiotic arrest in oocytes and the complex interplay of factors affecting chromosome segregation. The loss of sister chromatid cohesion is a key mechanism contributing to aneuploidy, especially with age. Studies in mice and humans have shown that cohesin loss is a significant factor in age-related aneuploidy. Environmental factors such as exposure to endocrine disruptors like bisphenol A (BPA) can also influence aneuploidy. BPA exposure has been linked to meiotic errors and altered oocyte development. Additionally, ART procedures may increase aneuploidy rates, highlighting the need for further research into the mechanisms underlying aneuploidy and the development of strategies to reduce its occurrence. Understanding these factors is crucial for improving reproductive health and addressing the challenges of aneuploidy in human pregnancies.