I have this dark area on my face that has been there since my last baby was born, three months ago. It won't go away. My mother says that it is mask of pregnancy and may never go away. Is that true?

Never Despair! Working with an excellent cosmetologist or dermatologist, you will be pleasantly surprised that may no longer be true. So many new products and different type laser techniques -you may find excellent success. You may wish to try a tiny isolated area first to be "sure" that tissue reaction is not as bad as the "melasma".
Mask of pregnancy. It may be just that. A dermatologist may be able to help, but generally it is hormonal and difficult to treat.