Complex sentence: A miscarriage is the natural death of a baby while it is still in its mother's womb, in the early stages of its development as a fetus prior to fetal viability (the stage of potential independent survival).
Split into simpler sentences: A miscarriage is the natural death of a baby while it is still in its mother's womb. Such a death takes place in the early stages of the baby's development as a fetus prior to fetal viability (the stage of potential independent survival).