Perhaps because it is normal for human embryos to contain cells with the wrong number of chromosomes, which can cause them to self - destruct. (newscientist.com)
As women age, their eggs are more likely to have the wrong number of chromosomes, which can lead to miscarriages. (newscientist.com)
When the dancers don't pair or part appropriately it can result in eggs and sperm with the wrong number of chromosomes, a major cause of miscarriage and birth defects. (sciencedaily.com)