Why do many severe chromosome abnormalities prevent development from continuing before birth?

Severe chromosomal abnormalities often prevent development from continuing before birth because they disrupt the fundamental genetic signals required for normal growth and biological function. When a conceptus possesses an abnormal chromosome number—such as an extra chromosome (trisomy) or a missing one (monosomy)—or structural defects like deletions, the resulting genomic imbalance is frequently lethal.123

In many cases, these genetic errors are incompatible with life, leading to the termination of development during the embryonic period. Because the vast majority of these severe mishaps occur very early, many pregnancies end in spontaneous abortion before a woman is even aware she is pregnant. While some chromosomal abnormalities, such as Down syndrome, are compatible with life, most autosomal trisomies and all autosomal monosomies are lethal in utero. Consequently, the abnormalities observed in living infants tend to be the milder types, as the most severe genetic disruptions typically prevent the fetus from reaching full-term birth.4563