B.N.
My friend's daughter did that. She started around 20 months and just stopped at 29 months (she was really nuts about it as she would pull out a chunk of hair, look at how much she pulled out and then often do it again). She is a VERY strong willed child and their doctor told them to calmly tell her that she needs to be nice to herself but not try to physically stop her as then it could be done just to get attention or become a control issue. Like with all kids they go thru phases (my son hit himself in the head for about 3 months - silly kids!!) and it will pass. Take a deep breath and know that you are a great mom and everything will be fine. I often have to tell myself - this too shall pass!