Our 18 mos old daughter did this, too. It completely freaked out both my husband and I since she started doing it around 6-8 months of age last yr right after autism awareness month and all we heard on TV were the signs of autism. Of course, twitching and repetitive motions were one of the signs. And of course it started on a weekend so I got on the internet which made me really concerned b/c there was so much on there related to neurological disorders. I took her to the pediatrician and she said it was completely normal. She had no other symptoms of autism or any other disorder and was developing normally.
Our daughter did this for probably 6 months. She seemed to do it more when she was watching you and if she got excited that really seemed to set it off. Sometimes she would do it with both hands and on occasion I would notice her doing it with her feet. Long story short, she was fine but it was just scary for me as a 1st time parent. If you are really worried go to the pediatrician but if your child has been otherwise healthy and maintains eye contact and reacts to your face then it's probably normal.
Hang in there, it will go away when they master their fine motor skills and begin walking and playing. I didn't notice it stop but it has, thinking back it was probably around 13 months after she had mastered walking.