Placing a pool of water in a birth room changes the atmosphere immediately. Voices get softer, the mother stays calmer and everyone becomes less stressed.
The effect of buoyancy that deep water immersion creates allows spontaneous movement of the mother. No one has to help the mother get into a new position. She moves as her body and the position of the baby dictate. Movement helps open the pelvis, allowing the baby to descend.
When a woman in labor relaxes in a warm deep bath, free from gravity’s pull on her body, with sensory stimulation reduced, her body is less likely to secrete stress-related hormones. This allows her body to produce the pain inhibitors-endorphins-that complement labor. Noradrenaline and catecholamines, the hormones that are released during stress, actually raise the blood pressure and can inhibit or slow labor. A laboring woman who is able to relax physically, is able to relax mentally as well. Many women, midwives, and doctors acknowledge the analgesic effect of water. Thousands of these mothers state they would never be able to consider laboring without water again.
from Waterbirth International