Birth Seasons. The birth of a new baby whale, known as a calf, usually occurs during the birth season for that specific type of whale. Orcas, for example, tend to give birth in the spring or fall, while blue and humpback whales do so in the winter. Gestation also varies from whale to whale -- with orcas it lasts around 17 months, with humpbacks 12 and with blues 11. Almost all whale births are to single calves, but rare twin births have been observed in wild whales.