Forgetting a birthdate is very common among mothers who have lost children to adoption, not only was it a very traumatic experience there was also a drug called Twilight sleep commonly used by doctors up to 1960, though uncommon it is still being used today by unethical doctors. here is a link if you are interested;
http://www.obgyn.upenn.edu/History/twilight.html
Some women are unaware of the drugs given to them at the time of birth. I had a very traumatic experience birthing my son, not only was I in labor for 49 hours I too was given so much moraphine I actually had to be woken from a drug induced sleep to push, held down by nurses so I would not look at my child before they wisked him out of the room, not to mention I was given drugs after delivery and did not wake up for nearly two days. I did not forget my sons birthdate but did block the memory of his birth up to four years ago. all this happened in 1988, if you can imagine alot of mothers in an earlier era were subject to barbaric horror's.
You may want to take a look into adoption practices in your era of birth you may find the reason behind your mothers lack of memory.