What prevent baby from breathing under water?

Human beings’ natural alliance with water is best witnessed in human babies who can swim naturally and easily long before they learn to sit up or crawl. During their first year of life, babies will calmly and happily paddle underwater, gazing around with eyes wide open. When they need to breathe, they naturally paddle toward the surface of the water before taking a breath. Babies instinctively know not to breathe while their heads are still submerged underwater. They wait until they reach the surface of the water before breathing. It seems to be only later that humans lose these instincts and become more prone to drowning.
There are several factors that prevent a baby from inhaling water at the time of birth. These inhibitory factors are normally present in all newborns. The baby in utero is oxygenated through the umbilical cord via the placenta, but practices for future air breathing by moving his intercostal muscles and diaphragm in a regular and rhythmic pattern from about 10 weeks gestation on.