Most species of baleen whales exhibit a seasonal breeding cycle, giving birth primarily during the winter and spring in sub-tropical to tropical breeding grounds. Giving birth in warmer water reduces the energy requirements on a newborn to stay warm while it develops an insulating blubber layer. Near-term pregnant females are usually the first to arrive in the calving grounds. C. Frequency of birth. Baleen whales give birth to a single calf.
Twins are extremely rare.
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 a limited milk supply from the mother. In most species, females may bear a calf every two to four years. Observations of baleen whale births at sea are extremely rare. The placenta seperates from the uterine wall during or soon after delivery.