Tokyo residents were warned on Wednesday not to give tap water to babies less than a year old because levels of radioactive iodine - which can cause thyroid cancer - are twice the recommended safe level in some areas of the city.
But Dr Joseph Gilhooly, a neonatalogist at Oregon Health and Science University in the US which has a water birth programme, said that babies who do not get enough oxygen during childbirth may gasp for air, risking allowing water to enter their lungs.