There's never a perfect time to have a baby, especially for the mammals inhibiting Earth’s most extreme winters.
Polar bears are solitary creatures, living across the vast, open icy plains of the Arctic region – so finding a member of the opposite sex is rare. So what does a female polar bear do if she mates outside of the right time frame? The solution is that polar bear females possess the physiological ability to store fertilized embryos in their bodies until their external environments become more suitable for newborns. This is called embryonic diapause, and it’s essentially a way for females to delay pregnancy. Embryos at the blastocyst stage enter a phase of dormancy, and they do not reanimate until they receive the appropriate physiological cues from a female's body. Diapause is common in mammals living in extreme or unpredictable environments; however, it’s hypothesized that all mammalian moms - even humans - may retain the ability to do it, even it’s rarely used.
For polar bears, it takes about eight months for baby to arrive once the mother initiates the gestational process. Females are most likely to find a mate between late March and June, and then give birth to one to three cubs in November or December. These polar mothers usually give birth while they are in their overwintering dens, and emerge with their cubs a few months after they’ve been born. Until that time, she uses her body fat stores to make enough milk to feed them.
Although they have the ability to time their emergence to coincide with the most favorable climatic conditions for finding enough food, polar moms have a tough journey ahead in order to successfully raise their cubs to sexual maturity. Conditions of the Arctic and subarctic regions remain unforgiving year round, and many large predators like wolves and even unfamiliar adult male polar bears are always on the hunt for easy food.
These polar mothers prove that in some climates there may never be the ‘perfect’ time to get pregnant. However, their bodies are evolutionarily predisposed to maximize their chances of success by storing embryos until the timing is the best it can possibly be. This, in addition to their attentive mothering for an extended period of time, gives these polar babies the best chance to reach adulthood.