When upright, the mother's body is in harmony with the downward pull of gravity. Less effort is demanded of the uterus. Since the uterus tilts forward during a contraction, a mother can lean forward and assist her uterus to work without resistance. When she is lying down, her uterus must work harder and less efficiently. It is easier for the baby's head to enter the mother's pelvis when she is upright, and provides the baby with the best angle of descent, in relation to gravity, through the pelvic canal.
Placental circulation is improved, giving the baby a better supply of oxygen.
The nerves that supply the pelvis and the uterus arise from the lower part of the spinal cord and enter through the sacrum. When there is no direct pressure on the nerves, less pain is felt. As long as the mother is upright, the pelvic joints are free to expand, move, and adjust to the shape of the baby's descending head. The coccyx is designed to swivel backwards to widen the outlet of the pelvis as the baby emerges. When a mother is upright, her perineal tissues can expand evenly and pull back around the baby's emerging head, significantly decreasing the need for an episiotomy.