1. Development indicators of weight and length The average weight of a three-month-old baby is 5.7-6.5 kg and the length is 58-64 cm. But it’s important to note that every baby grows at a different rate, so this is just a general range. If your child falls outside this range, there is no need to panic, as long as he/she is healthy and happy. 2. Development indicators of head circumference and chest circumference The average head circumference of a three-month-old baby is 39.5-41 cm, and the chest circumference is 38-40 cm. Head circumference and chest circumference are also important indicators for measuring the healthy growth of babies.