The mating system of narwhals is unknown.

Monodon monoceros is a seasonal breeder. The gestation period is about 15.3 months, with mating occurring in March-May and calving in July-August of the following year. Lactation duration is unknown, but thought to be comparable to the white whale (Delphinapterus leucas) of 20 months. The interval between successive conceptions is normally three years.  Monodon monoceros copulate vertically in the water, belly to belly. Infant narwhals are usually implanted in the left uterine horn. A single calf is often the result of gestation, yet some twins have been recorded. Birth takes place tail first (Klinowska, 1991). The newborn is born with 25 mm of blubber. Calves usually measure between 1.5 and 1.7 m and weigh 80 kg. Physical maturity is attained at a length of 4 m and a weight of 900 kg in females and 4.7 m and 1600 kg in males. This usually corresponds to 4 to 7 years of age (Reeves & Tracey, 1980).

Breeding season: March to May

Range number of offspring: 1 to 2.

Range gestation period: 12 to 15.3 months.

Average gestation period: 13 months.

Range weaning age: 12 to 24 months.

Range age at sexual or reproductive maturity (female): 4 to 7 years.

Range age at sexual or reproductive maturity (male): 4 to 7 years.

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

Average birth mass: 80000 g.

Average number of offspring: 1.

Young narwhals are capable of swimming soon after birth. They are nursed and protected by their mothers for extended periods after birth.

Parental Investment: precocial ; pre-fertilization (Provisioning, Protecting: Female); pre-hatching/birth (Provisioning: Female, Protecting: Female); pre-weaning/fledging (Provisioning: Female, Protecting: Female); pre-independence (Provisioning: Female, Protecting: Female); extended period of juvenile learning