Mating takes place on the sea ice from April until late June, but the fertile ova do not implant until the following fall. This is called delayed implantation.
Females usually give birth to cubs about two months after they enter the maternity den, usually by early January.
- Newborns are 12 to 14 inches long (30–35 cm) and weigh little more than one pound (0.5 kg). Cubs grow rapidly on their mother's rich milk.
- The family remains in the den until March or early April. During her entire time in the den—four to eight months—mother bear doesn't eat or drink. When she finally emerges with her cubs, she leads them to the sea ice so she can break her long fast by hunting seals.
For at least 20 months, cubs drink their mother's milk and depend on her for survival. Her success at hunting is critical for her own needs and for teaching cubs to find food for themselves.
Today, cubs generally stay with their mother for two-and-a-half to three years. Historically, polar bears in Hudson Bay weaned in half that time due in part to the high productivity of that region.This is becoming less frequent as that system changes.