Chromosomal abnormalities are a common culprit in miscarriage and stillbirth, but you might be wondering why that is the case. Given that many babies can go on and be born with genetic conditions such as Down Syndrome or other trisomies, why is it that some chromosomal abnormalities lead to miscarriage?
In most instances, scientists do not know the exact reason why chromosomal abnormalities lead to miscarriage. One theory is that the mother's immune system recognizes a problem in the developing baby's genes and thus ends the pregnancy. If you hear the (somewhat insensitive) explanation that miscarriages are "nature's quality control," the person saying this is going by the theory that a mother's immune system terminates pregnancies affected by chromosomal abnormalities.
Another theory is that the developing baby ultimately reaches a point where the specific genetic problem causes the baby to stop growing. Certain genes might be missing that are necessary for continued development, or extra copies of certain genes (or chromosomes) might cause the baby or placenta to not grow properly. This could explain why some certain types of chromosomal abnormalities lead to miscarriage while others do not.
More research may ultimately reveal the exact mechanism by which chromosomal issues cause a miscarriage.