A birthing ball or labor ball is a large air–filled rubber ball (a gym ball about 70 centimetres (28 in) in diameter) that a woman can sit on during labor. A birthing ball allows the woman to rock back and forth seated on a softer surface. A woman on a birthing ball may need a support person to keep her steady. A birthing ball may be especially useful during back labor. Hospital policies on birthing balls vary, in part because the balls provide a somewhat unsteady seat, some hospitals are concerned about their safety if a woman falls during birth. It is often used during natural childbirth.