Aneuploidy often causes birth defects and miscarriages. Birth defects make up about 3% of births in the United States. Among those who survive birth, Down syndrome is the most common form of aneuploidy. Many with Down syndrome do survive to adulthood. Some cancer cells also have abnormal numbers of chromosomes. The aneuploid theory of cancer suggests that aneuploidy is a cause of cancer rather than a result of it. 