This is one of the biggest misconceptions about teaching a hearing baby to sign. It’s also the one that most people just toss out willy nilly with no thought whatsoever. I’m harsh with this, but only because I’ve heard it so often and it’s frequently motivated by negative thoughts about signing. Think competitive parents, disapproving in-laws and the like. As reasonable as the comment sounds at first impression, with a little bit of critical thought it’s pretty obvious that there’s no way to silence a baby. Talking is innate and only very severe traumatic events can produce mute child. This is a nasty thought, but it sure helps enlighten a naysayer. Far from traumatic, signing is an excellent way to connect with your baby in a constructive and loving way.
Sign language doesn’t inhibit speech development in any way. Think of signing like crawling on the way to walking, except in this case, it’s on the way to talking. Your baby is going to drop signing as soon as they are no longer useful. There will be a period where your baby uses both sign and words at the same time, but quickly they will learn that signing has some major draw backs like it only works face to face and that it’s hard to do things with busy hands and sign at the same time.