Although Junior's only tipping the scales around 3.5 ounces, that's a significant gain from just two weeks ago, when he only weighed half as much. He's now starting to pack on the pounds as fat is forming in his body. Roughly 5 inches from rump to crown, your baby's skeletal system is hardening from cartilage into bone, and his urinary and circulatory systems are fully functional. He's also reached the stage of development where he can hear. His world is full of many sounds, from your incredibly loud heartbeat to the rumblings of your active digestive system.