A typical den has an underwater entrance hole, a living space above water level and several air or exit-entry holes to dry ground. Otters mature sexually by two years of age. They breed sometime between January and May, mating taking place in the water. As with many other mustelids, otters have delayed implantation. This means that after fertilization, eggs remain dormant in the female's uterus until the following December, January or February, when they attach to the uterine wall and start to develop.
Females usually have their first litter at age three.
Pups weigh 4-5 ounces and are blind and toothless at birth. They do not open their eyes for five weeks, and their mother keeps them in the den until they are three or four months old. The female breeds shortly after giving birth, but she will not allow a male near her young for several months. Males are polygamous. When a young otter emerges from the den, its mother must teach it to swim.