Breast growth is usually the first sign of puberty girls will notice. First, small lumps form behind the nipples. They can be sore, but the pain goes away as breasts grow and change shape over the next few years. As they grow, it’s not unusual for one breast to develop more slowly than the other, but they will even out in time. Boys also may have some swelling on their chest but it tends to go away within a year or two.