This is an individual developmental milestone for our babies. Usually around 6 months, a child will be happy moving to your back. However, it is not unheard of that a 3-½ month old baby can be very happy on the back as well. If a child is fussing in the back carry position for the first time, bringing them to a mirror, where they find comfort in seeing themselves, will often settle them. Also, bouncing and movement may help.