During mating season, males and females may form a social unit, but strong data on this issue are lacking.

Mating occurs during the winter months. Sei whales in the Northern Hemishpere mate between November and February, whereas mating in the southern hemisphere occurs between May and July. Gestation lasts from 10 1/2 to 12 months. Females typically give birth to a single calf measuring 450 cm in length. There are reports of rare multiple fetuses. The calf nurses for six or seven months. Young reach sexual maturity at 10 years of age, but do not reach full adult size until they are about 25 years old. Sei whales may live as long as 74 years.

Females typically give birth every other year, but a recent increase in pregnancies has been noted. Researchers think this may be a response to the predation rate. Humans kill a great many whales each year, and this might have effects on their reproductive activity.

Breeding interval: Females typically give birth every other year

Breeding season: Mating occurs during the winter months

Average number of offspring: 1.

Range gestation period: 10.5 to 12 months.

Range weaning age: 6 to 7 months.

Average age at sexual or reproductive maturity (female): 10 years.

Average age at sexual or reproductive maturity (male): 10 years.

Key Reproductive Features: iteroparous ; seasonal breeding ; gonochoric/gonochoristic/dioecious (sexes separate); viviparous

Average birth mass: 680000 g.

Average number of offspring: 1.

Average age at sexual or reproductive maturity (male)

Sex: male: 3652 days.

Average age at sexual or reproductive maturity (female)

Sex: female: 3652 days.