"The most common cause of miscarriage is chromosomal/genetic abnormalities, leading to ~50% of spontaneous miscarriages. Genetic abnormalities most frequently occur due to aneuploidy—when the chromosomes fail to separate properly during cell division—meaning that some cells have extra copies of a particular chromosome while others have none. The exact causes of aneuploidy are not well understood, although increased maternal age (>35 yrs) is the greatest risk factor."