a specfic mutation in a certain animal leads to abnormal development of the heart and kidney, resulting in miscarriage. Why did this happen?

These kinds of mutations are referred to as Lethal mutations

Explanation:

This type of mutation deleteriously affects the vital genes that are responsible for the core functions of the body. The heart supplies blood to the body organs for nutrients and oxygen while kidneys filter out toxins and end products of metabolism. Mutations in genes for development of these organs will be deadly.