Final answer:
The heaviest newborn organ is the liver, which is the largest gland in the body and plays important roles in metabolism and regulation.
Step-by-step explanation:
The heaviest newborn organ is typically the liver. The liver is not only the largest gland in the body but it also plays a multitude of crucial roles in metabolism and regulation. In an adult, it weighs about three pounds and although the liver of a newborn is proportionately smaller, it is still substantial in size relative to the body.
While the brain is the most complex organ and becomes a significant consumer of the body's energy as it develops, it is not the heaviest at birth. The heart is the first organ to function, pumping blood as early as 21 to 22 days after fertilization, but it does not outweigh the liver at birth.