Polar Bear Gestation Calculator

Polar bear pregnancy technically spans 195 to 265 days from mating to birth, but actual embryo development only takes 60–70 days — the rest of the time is delayed implantation, when the fertilized egg floats freely without developing.

After mating in spring, the fertilized egg does not implant in the uterine wall right away — it floats freely in a dormant state for months. Implantation only occurs in autumn, and only if the female has accumulated enough fat reserves from summer hunting. If she has not gained sufficient fat, her body reabsorbs the embryo entirely and the pregnancy simply ends without ever fully beginning.

Pregnant females dig snow dens, typically in October or November, and give birth in December or January while still inside the den. The mother and newborn cubs remain denned through the depths of winter, emerging together in March or April once the cubs are strong enough to survive the Arctic cold outside.

Polar bear cubs are born astonishingly tiny — around 500 grams, roughly 1/100th of their mother's weight. This extreme size difference between newborn and mother is unusual among large mammals and reflects how much of a cub's growth happens after birth rather than in the womb.

No — a pregnant female polar bear does not eat at all for roughly 8 months, from entering her den in October through emerging with cubs in March or April. She survives entirely on stored body fat while also producing rich milk to nurse her cubs, making pre-denning fat accumulation absolutely critical to her and her cubs' survival.

Polar bear litters typically contain 1 to 3 cubs, averaging around 2. Twins are the most common outcome when the mother is in good body condition; single cubs and, more rarely, triplets occur depending on her fat reserves and overall health going into the den.

Sea ice loss is shortening the polar bears' hunting season, since they rely on sea ice to hunt seals, their primary prey. Females are increasingly arriving at denning sites too thin to sustain a pregnancy through the winter fast, leading to lower cub survival rates and, in some populations, declining birth rates — one of the clearest documented links between climate change and a specific species' reproductive success.

At roughly 240 days average (with the same delayed-implantation mechanism), polar bear gestation is close to brown bears (220 days) and black bears (220 days), all of which share the delayed implantation strategy common across most bear species.