Females usually have their first calf at about 10 years of age. A single calf is born at a time, rarely two, with a gestation period of 227-240 days. The calf weighs 60-100 pounds at birth and measures up to 3 feet long. Calves are sometimes born underwater and must swim to the surface for their first breath of air. They also nurse underwater some of the time.
Young hippos occupy most of their time in the water standing on the backs of their mothers.
Otherwise, the effort to keep afloat would wear them out.