It is well documented that water can help with labour pains. It works by the heat of the water reducing muscle spasm and the buoyancy of the water relieving pressure on the pelvis – this lessens the overall pain experienced. The water is kept around body temperature by topping up with warm water and needs to be covering the ‘bump’ to be effective. Studies have shown that it can reduce the length of labours and the risk of tearing. Babies are born completely under water so that they do not gasp until they hit the cold air. Most hospitals will allow you to use ‘gas and air’ in the pool as an additional form of pain relief, if the water is not enough.
See: Forde et al (1999) Labour and Delivery in the birthing pool. British Journal of Midwifery 7(3): pp165-71.