The most remarkable fact about human fertility is that up to 85 percent of all miscarriages are due to chromosomal abnormalities, meaning that the new embryo has extra, missing, or badly broken chromosomes. Doing the math, this means that when a human sperm and a human egg fuse, the resulting embryo ends up with the proper number of intact chromosomes only around two-thirds of the time. The remaining 15 percent of miscarriages are caused by a variety of congenital conditions, such as spina bifida or hydrocephalus.
