Most species of baleen whales give birth seasonally - primarily during the winter, as they approach warm-water, low-latitude breeding grounds or after they arrive. In the warmer-water breeding grounds, a newborn whale uses far less energy for keeping warm than it would in polar seas. (It quickly develops the insulating blubber layer that will help it maintain body temperature when it migrates toward colder seas in the spring.)

Baleen whales give birth to a single calf. In most species, a female may bear a calf every two to four years. If twins are conceived, they are unlikely to survive to full term. In rare cases where twins are born, they are not likely to survive due to the limited milk supply from the mother.

Observations of baleen whale births are extremely rare.

Some species seek protected coastal areas to give birth.

Tail-first deliveries have most often been observed, and a few head-first deliveries have also been documented.

Whale calves can swim at birth.

In general, a calf is approximately a quarter of the mother's length. A calf's birth weight is about 3% to 4% of its mother’s weight.

Baleen whale calves have small, soft baleen plates that soon stiffen.

A whale calf suckles from nipples concealed in its mother's abdominal mammary slits. Calves nurse under water.

A baleen whale calf nurses for 4 to 11 months. By its first summer in feeding grounds, it is weaned.

The high fat content (up to 40% to 50% fat) of whale milk allows the calf to rapidly develop a thick insulating layer of blubber.

In general, whale calves gain weight quickly, although growth rate varies by species. For example, nursing blue whale calves gain 90 kg (200 lb.) each day. Gray whale calves double their weight in about three months and double their length in about two years.

Sei whales are 4.5 m (15 ft.) at birth and grow about 2.5 cm (1 in.) each day. Nursing humpback whale calves grow 45 cm (1.5 ft.) per month.