Female giant pandas raise their young alone without the involvement of other adults, and while they may give birth to two cubs at once, there is usually only one survivor. Cubs are born small and helpless, unable to move until 3 months of age, and depend on their mother's milk for nourishment and on her strength for protection from predators.
Pandas nurse their cubs for 8 or 9 months, sometimes up to a year, but the cub remains with its mother for 2 to 3 years. This makes pandas very slow to breed, and their short periods of ovulation further complicate this problem so that some pandas only raise five to eight cubs over the course of their lives.
A mother panda must forage for long periods of time in order to produce the milk her cub needs. Pandas already spend a huge percentage of their time grazing for food, so this added workload and responsibility can be very difficult for the animals.
Panda pregnancies last 95 to 160 days and result in the birth of tiny cubs that are 1/900th their mothers' size. They grow slowly, beginning their lives no larger than sticks of butter before developing into full-grown bears over the years.