Female black bears in the Sierra Nevada are usually between three to five years of age when they have their first litter. Courtship and mating usually takes place in a two or three week period in June, July and sometimes as late as August. Bears are polygamous and a single male may mate with several females. The gestation period for black bears is 235 days, but embryonic growth only takes about two months. This is because bears have a unique reproduction system called embryonic delay, or delayed implantation. Although mating takes place in the summer, the fertilized ovum does not implant for many months. Because of this wonderful physiological mechanism, bears are only able to give birth when they are in good condition. If the sow is in poor condition, due to inadequate food sources, and has not gained enough weight to support herself and her cubs during the winter, she will abort, and the blastocyst is absorbed by her body.

If a sow is healthy enough, she will give birth during hibernation inside the den in late January or February. Average litter size is two cubs; however, three cubs are not uncommon in the Sierra Nevada. Cubs weigh about 8 ounces at birth and grow to approximately 10 to 12 pounds by the time they leave the den three months later. The cubs will stay with their mother for the rest of the year and will den with her the next winter. After emerging from the den, they will stay as a family unit until the sow is ready to mate again in early summer.