Beaver mate and have young when three years old. Breeding takes place in January and February. The period of gestation is thought to be between three and four months. The young are born in May and June. The average number of young per litter is four.
Older females tend to produce larger litters than young females.
At birth a young, termed a kit, weighs one pound or slightly less. Kits have downy fur, open eyes, and are able to swim a short time after birth. Although the young are able to swim at once, they seldom come out of the den until about 1 month old. Then they swim with their mother who often carries them on her back in the water. The female takes entire care of the kits until this time, but the male soon returns to the family.