From the studies I've seen, signing with babies who have normal hearing doesn't appear to have a negative effect on language development. In fact, some studies indicate that it may actually boost verbal skills. However, teaching your baby to sign takes time and a commitment to learning and consistently using the signs yourself in order to see desired results: A baby between 10 and 14 months using specific gestures to convey his wants and needs.
Interestingly, signing may be beneficial because parents who sign with their baby are spending more time focusing on communicating -- observing their child and trying to understand what he is saying, as well as reinforcing communication attempts their child makes. In addition, these parents may be using a technique called elaboration: When a baby makes a sign for "more," for instance, the parent may respond, "You want more juice. I'll put some more in your cup."
But you don't need to institute a formal signing program with your baby to have these enriching conversations in which you read your baby's cues and respond through both words and actions. If you watch your baby carefully, you will see that she is signaling to you all the time. When you play peekaboo and you stop, she reaches out and babbles to let you know she wants you to keep playing. When she raises her arms to you, she is telling you she wants you to pick her up. And she'll cry and bang on the high chair to let you know you're not bringing the food fast enough. Responding to these kinds of cues promotes her language skills (along with her social and emotional development). So, whether you choose to try formal sign language with your child is your call. The important thing is to keep exposing your daughter to all forms of language -- talk with her, elaborate on what she tries to tell you, let her know you understand her, sing to her, and explore books together.
Originally published in American Baby magazine, February 2004. Updated 2009.