Estimates of sexual maturity in baleen whales range from about 4 to 11 years, depending on the species. This information is not known for all species.
Sexual maturity is often reported as length. For example, Bryde's whales are sexually mature at about 12 m (39 ft), while minke whales are mature at about 7–8 m (23–26 ft).
Mating and birth seasons are linked to the annual migration cycle. While there is much variation, many whales mate in low-latitude (tropical) calving areas during the winter.
Female baleen whales probably come into estrus (the period of sexual receptivity) once a year, but if conception fails to occur, they may come into estrus a second time.
Male display a seasonal reproductive cycle which correlates with a winter estrus in females.
In right, bowhead, and gray whales, breeding behavior often includes several males and one female. These breeding groups may interact for more than an hour before mating. The female may allow the largest and most aggressive male to mate with her, or she may flee or roll away. A female may mate with more than one male during a single breeding season.
Right whales and rorqual whales engage in aggressive breeding activity that includes males fighting with each other to gain access to a female. Battles result in males ramming and sometimes wounding each other. Females may mate with just one winning male.
Male humpback whales are famous for their "songs," which have been noted only in winter mating grounds. Whether or not these songs play a role in mating activity is not known.
Gestation period for baleen whales ranges from about 10 to almost 14 months, depending on the species.
A pregnant female's energy needs greatly increase during the second half of the gestation period. Also, a pregnant female stores energy that she will need to lactate (produce milk) after her calf is born. In some species, pregnant females in their last six months of pregnancy eat as much as 50% to 60% more than non-pregnant females.
Newly pregnant females are usually first to leave the breeding grounds and head for summer feeding grounds. They are usually last to leave feeding areas in the fall.