1. Physical development standards for three-month-old babies 1.1 Weight and height At three months of age, the average baby should weigh about 12 pounds (5.4 kilograms) and be about 24 inches (61 centimeters) long. During this period, the baby\’s weight growth will slow down compared to when he was born, but his height will continue to grow. 1.2 Head circumference and chest circumference When the baby is three months old, the baby\’s head circumference and chest circumference will also increase. The average head circumference is 15