Baby started this habit at around 7-8 months where she rocks front to back in her high chair.
She also does it just before sleeping, and right after she wakes up.
It is very disconcerting to watch, since she bangs her head/body quite hard. The high chair has padding, so she bumps away with her head and back, hard enough to make the chair back up until it stops at the wall.
In her crib, she learned that crib bars can hurt, so she tucks in her chin and carries on merrily. I think she stops just short of hurting herself. I constantly check her for bruises, but she seems ok.
When she rocks while awake, I try to distract her, or just wait till it passes. NEVER try to feed a rapidly moving toddler in a high chair - unless you feel like wearing most of what you are feeding her.
She is 13 months old and I checked with the family doctor at her 9-month and 12-month appointments, since rocking can also be symptoms of something else. Everything seems ok, so I am still worried, but relieved.
I had the toughest time finding any information on the Internet. Try typing "rock" and "baby" and you'll see what I mean.
I thought I would post a couple of useful links I found in case any other parents.
Apparently, this type of rocking back and forth is common for children under 3 years old and they will grow out of it. But, I suggest to any parent to confirm with their family doctor - if there are other symptoms, it could mean something else.
http://www.babycenter.com/0_sleep-co...ing_1286134.bc
http://www.med.umich.edu/1libr/yourchild/badhabit.htm
http://raisingchildren.net.au/articl...d-banging.html
http://www.kidsbehaviour.co.uk/Child...dyRocking.html