Hibernation. With the exception of pregnant females, polar bears do not overwinter in dens like brown and black bears. Instead adult male polar bears and non-pregnant females remain active throughout the year.
Pregnant female polar bears dig a snow den, give birth, and emerge three months later. During this time, they live off their fat reserves. But they don't hibernate in the strict sense of the word.
True hibernators experience a marked drop in heart rate and body temperature. Mother bears do not enter a state of deep hibernation because they need a higher body temperature in order to meet the demands of pregnancy, birth, and nursing.
Denning. Pregnant polar bears den in the fall after feeding heavily in August and September. They choose den sites in snowdrifts along mountain slopes or hills near sea ice or in banks of snow on the frozen sea. Along Western Hudson Bay mother bears begin by denning in earthen dens along river banks and later move to snow dens.
To build her den, the female scrapes a tunnel into the snow and digs two chambers. She gives birth to one, two, or three cubs in November or December. Twins are most common.
The family emerges from the den in March or April when the cubs are strong enough to make the trek to the sea ice.