<P> Most human babies demonstrate an innate swimming or diving reflex from birth until the age of approximately six months . However, babies this young cannot actually swim, due to their lack of body features and strength . Other mammals also demonstrate this phenomenon (see mammalian diving reflex). This reflex involves apnea (loss of drive to breathe), slowed heart rate (reflex bradycardia), and reduced blood circulation to the extremities such as fingers and toes (peripheral vasoconstriction). During the diving reflex, the infant's heart rate decreases by an average of 20% . The glottis is spontaneously sealed off and the water entering the upper respiratory tract is diverted down the esophagus into the stomach . The diving response has been shown to have an oxygen - conserving effect, both during movement and at rest . Oxygen is saved for the heart and the lungs, slowing the onset of serious hypoxic damage . The diving response can therefore be regarded as an important defence mechanism for the body . </P> <P> Drowning is a leading cause of unintentional injury and death worldwide, and the highest rates are among children . Overall, drowning is the leading cause of injury death among children aged 1--4 years in the United States, and is the second highest cause of death altogether in that age range, after congenital defects . </P> <P> A Centers for Disease Control and Prevention study in 2012 of United States data from 2005--2009 indicated that each year an average of 513 children aged 0--4 years were victims of fatal drowning and a further 3,057 of that age range were treated in U.S. hospital emergency departments for non-fatal drowning . Of all the age groups, children aged 0--4 years had the highest death rate and also non-fatal injury rate . In 2013, among children 1 to 4 years old who died from an unintentional injury, almost 30% died from drowning . These children most commonly drowned in swimming pools, often at their own homes . </P> <P> Traditionally, swimming lessons started at age four years or later, as children under four were not considered developmentally ready . However, swimming lessons for infants have become more common . The Australian Swimming Coaches and Teachers Association recommends that infants can start a formal program of swimming lessons at four months of age and many accredited swimming schools offer classes for very young children, especially towards the beginning of the swimming season in October . In the US, the YMCA and American Red Cross offer swim classes . A baby has to be able to hold his or her head up (usually at 3 to 4 months), to be ready for swimming lessons . </P>

When do babies learn to hold their breath