Gestation periods range from 10 to almost 14 months depending on the species and are linked to the annual migration cycle. A pregnant female's energy needs increase greatly during the second half of the gestation period.
Excess energy is stored until it is spent during lactation.
Newly pregnant females are usually the first to leave the breeding grounds and first to arrive in the summer feeding grounds. B. Birth seasons. 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.