"If Dr. Maria Montessori were alive, would she suggest baby sign language?" I used to ask myself that question a lot! I do not think she would be against it since she worked with handicapped and special needs children early on in her career. But I don’t think she would have included it into her infant toddler curriculum, either—just a hunch.
I was opposed to it for my son, who has Down syndrome until I watched a DVD for parents of young children with Down syndrome. And it had to do with auditory skills.
For example, if I were to say to my son, “juice,” he might hear “juh” or “u” or “s” but not “juice.” Now, let’s say I hold up a cup of his juice and I sign “juice,” he would understand me by seeing the sign and the juice, but he probably would not hear the word correctly. (It later turned out my son had hearing loss.)
So now you sign to him “juice,” and he has a mental picture of “juice” and can sign it back.
After I watched this DVD and they talked about sign language, and how I understood it for my son, I was convinced it was the right choice for my son. However, he did not show an interest in baby signs (and sign language) until age 2½.
I do not think you can harm your baby’s language development by using sign language.
If you want to teach signs, use the concrete object as much as possible (I think Maria Montessori would agree with that).
We used the Signing Time DVD series to teach our son sign language, he loved each and every one! He learned over 50 signs (I lost count after a while) including the signs for the letters of the alphabet! I LOVED these DVDs, too, because they taught me sign language as well!
You can also watch YouTube sign language video clips to learn all the different signs.