When the ice floes are at their minimum in the fall, ending the possibility of hunting, each pregnant female digs a maternity den consisting of a narrow entrance tunnel leading to one to three chambers.
Most maternity dens are in snowdrifts, but may also be made underground in permafrost if it is not sufficiently cold yet for snow.
In most subpopulations, maternity dens are situated on land a few kilometres from the coast, and the individuals in a subpopulation tend to reuse the same denning areas each year.
The polar bears that do not den on land make their dens on the sea ice.
In the den, she enters a dormant state similar to hibernation.
This hibernation-like state does not consist of continuous sleeping; however, the bear's heart rate slows from 46 to 27 beats per minute.
Her body temperature does not decrease during this period as it would for a typical mammal in hibernation.

Between November and February, cubs are born blind, covered with a light down fur, and weighing less than 0.9 kg (2.0 lb), but in captivity they might be delivered in the earlier months.
The earliest recorded birth of polar bears in captivity was on 11 October 2011 in the Toronto Zoo.
On average, each litter has two cubs.
The family remains in the den until mid-February to mid-April, with the mother maintaining her fast while nursing her cubs on a fat-rich milk.
By the time the mother breaks open the entrance to the den, her cubs weigh about 10 to 15 kilograms (22 to 33 lb).
For about 12 to 15 days, the family spends time outside the den while remaining in its vicinity, the mother grazing on vegetation while the cubs become used to walking and playing.
Then they begin the long walk from the denning area to the sea ice, where the mother can once again catch seals.
Depending on the timing of ice-floe breakup in the fall, she may have fasted for up to eight months.
During this time, cubs playfully imitate the mother's hunting methods in preparation for later life.

Female polar bears have been known to adopt other cubs.
Multiple cases of adoption of wild cubs have been confirmed by genetic testing.
Adult bears of either gender occasionally kill and eat polar bear cubs.
As of 2006, in Alaska, 42% of cubs were reaching 12 months of age, down from 65% in 1991.
In most areas, cubs are weaned at two and a half years of age, when the mother chases them away or abandons them.
The Western Hudson Bay subpopulation is unusual in that its female polar bears sometimes wean their cubs at only one and a half years.
This was the case for 40% of cubs there in the early 1980s; however by the 1990s, fewer than 20% of cubs were weaned this young.
After the mother leaves, sibling cubs sometimes travel and share food together for weeks or months.
When the ice floes are at their minimum in the fall, ending the possibility of hunting, each pregnant female digs a maternity den consisting of a narrow entrance tunnel leading to one to three chambers. Most maternity dens are in snowdrifts, but may also be made underground in permafrost if it is not sufficiently cold yet for snow. In most subpopulations, maternity dens are situated on land a few kilometres from the coast, and the individuals in a subpopulation tend to reuse the same denning areas each year. The polar bears that do not den on land make their dens on the sea ice. In the den, she enters a dormant state similar to hibernation. This hibernation-like state does not consist of continuous sleeping; however, the bear's heart rate slows from 46 to 27 beats per minute. Her body temperature does not decrease during this period as it would for a typical mammal in hibernation.
Between November and February, cubs are born blind, covered with a light down fur, and weighing less than 0.9 kg (2.0 lb), but in captivity they might be delivered in the earlier months. The earliest recorded birth of polar bears in captivity was on 11 October 2011 in the Toronto Zoo. On average, each litter has two cubs. The family remains in the den until mid-February to mid-April, with the mother maintaining her fast while nursing her cubs on a fat-rich milk. By the time the mother breaks open the entrance to the den, her cubs weigh about 10 to 15 kilograms (22 to 33 lb). For about 12 to 15 days, the family spends time outside the den while remaining in its vicinity, the mother grazing on vegetation while the cubs become used to walking and playing. Then they begin the long walk from the denning area to the sea ice, where the mother can once again catch seals. Depending on the timing of ice-floe breakup in the fall, she may have fasted for up to eight months. During this time, cubs playfully imitate the mother's hunting methods in preparation for later life.
Female polar bears have been known to adopt other cubs. Multiple cases of adoption of wild cubs have been confirmed by genetic testing. Adult bears of either gender occasionally kill and eat polar bear cubs. As of 2006, in Alaska, 42% of cubs were reaching 12 months of age, down from 65% in 1991. In most areas, cubs are weaned at two and a half years of age, when the mother chases them away or abandons them. The Western Hudson Bay subpopulation is unusual in that its female polar bears sometimes wean their cubs at only one and a half years. This was the case for 40% of cubs there in the early 1980s; however by the 1990s, fewer than 20% of cubs were weaned this young. After the mother leaves, sibling cubs sometimes travel and share food together for weeks or months.
Polar bear