Calves are born mid-April to mid-June, during the northward migration.

Calves are usually born on the ice.

A female generally gives birth to a single calf at a time. Twins are rare, although they have been reported.

Newborn calves weigh about 45 to 75 kg (99-165 lb.) and are about 95 to 123 cm (3-4 ft.) long.

Calves are ashen gray to brown with dense, short soft fur. About two to three months before birth, the calf sheds a fine white layer of soft fetal hair called lanugo.

Nursing.

Cows with calves more than two days old tend to gather in herds separate from the bulls and other females. These "nursery herds" usually include 20 to 50 individuals but may include as many as 200.

A cow is extremely protective of her calf. She defends and protects her calf and may shelter it under her chest between her foreflippers. Calves often ride on their mothers' backs in the water.

There is some evidence that females may care for orphans, although it is unknown whether the female nurses the orphan.

The calf grows about 10 to 15 cm (4-6 in.) in length each month. In a zoological environment, a calf gains 0.7 to 0.9 kg (1.5-2 lb.) per day. Males grow slightly faster than females.

By one month of age calves are strong swimmers.

A pregnant, near-term cow and her calf from the previous pregnancy usually separate in late April, just before the new calf is born. Mother and calf stay together two years or longer if the mother does not produce another calf. Females usually remain with their mother's herd. Young males may stay an additional two or three years before joining an all-male herd.