MODEL RELEASED. Pregnant woman sitting on a large inflatable rubber ball, known as a birthing ball, that is identical to and has a similar function to exercise balls. Sitting on the ball is thought to encourage a posture that helps the baby settle into the correct position for birth. The ball is also used as an aid for prenatal exercises. This woman is 30 weeks pregnant. A full-term pregnancy lasts between 37 and 42 weeks.