Polar bear cubs stay with their mothers and are nursed for up to 30 months. However, most cubs are weaned and out on there own at about 2 years of age.
During pregnancy, female polar bears begin by depositing fat in preparation for cubbing. In order to have a successful pregnancy, they need to gain at least 200 Kg (441 lbs). They then seek out a den to give birth in. The dens protect the newborns from predators and the harsh environment outside of the den.
Most cub litters are 2 cubs followed by a single cub being born. Litters of 3 and 4 are very rare. At birth, polar bear cubs are very small and weigh between 454 and 680 grams (16-24 oz). They are born with their eyes closed and only have a fine, thin hair on their bodies, which makes them seem hairless at birth.