Ready for a little gymnast? "By four months of age, the baby should be able to hold her head, chest, and arms up when laying on the stomach," Dr. Jacobson said. "The baby should start swatting at things, kicking the baby gyms, and holding a rattle." The baby might also roll over, but not all babies roll over at four months, according to Dr. Jacobson. "When the baby sits up with support at four months of age," she continued, "the head can fall forward. Many parents place the baby in a Boppy pillow or just put pillows around the baby so if she falls over, she is not injured."