Journey North Information About Humpback Whales Life Cycle How often do humpbacks give birth? Most females give birth every two or three years, but several females have borne calves annually. Do humpbacks ever have twins? Humpbacks and other whales have only one baby at a time. A whale has never been seen with two live young, however, there were reports during whaling days of a pregnant whale with two fetuses. What is their gestation period? Pregnancy lasts for 11 to 12 months. How big are the calves when they are born? The calves are usually about 13 to 15 feet (3.75-4.5 meters) long at birth. They nurse for up to a year and grow about 1.5 feet (45 cm) per month. Does the mother stay with the calves for a long time? Mothers stay with their calves at least a year. They are very protective of their calves. A mother will not abandon her calf, even in extreme danger. Early whale hunters took advantage of this behavior, first killing a calf, then killing the mother, who refused to leave the carcass. Copyright 2001 Journey North. All Rights Reserved. Please send all questions, comments, and suggestions to our feedback form