Miscarriages must occur because our DNA is constantly mutating. Some of those mutations will be so harmful that the new organism cannot even survive until birth

Miscarriages must occur because our DNA is constantly mutating. Some of those mutations will be so harmful that the new organism cannot even survive until birth. In addition to those unavoidable ones, there can also be things going wrong by accident (e.g. the pregnant woman suffer physical injury which kills the fetus).