During the third month, the fetus definitely resembles a human, but the head is relatively large. During the ensuing months, the remainder of the body increases in size proportionally. Cartilage is replaced by bone, and the reproductive organs develop.
During the fourth month, the length of the fetus increases to about 6 inches. The heartbeat can be heard through the mother’s abdominal wall, and the fetus moves about. Distinctive movements can be felt at the fifth month, and by the sixth month the fetus weighs almost 2 pounds. By the end of six months, the fetus might be able to survive outside the mother’s body, but it would have little fat in its skin, so temperature control would be a problem. By the end of the ninth month, the fetus has an average length of about 20 inches and a typical weight of 6 to 8 pounds.