Until the mother begins her winter hibernation, grizzly bear embryos do not begin to develop, although mating may have taken place up to 6 months before. As with other bears, if the mother has not accumulated enough fat to sustain herself as well as developing cubs, the embryos may not implant. In January, usually one to three cubs, each weighing only a pound or less, are born. The cubs gain weight quickly and by the time they come out of the den, they often have reached 20 pounds. As many as half of all cubs may not reaching breeding age - a leading reason for the grizzly's low numbers.