Every child is different and there is a huge amount of difference in speech development for lots of children. This is from a website I frequent "He'll begin by using his tongue, lips, palate, and any emerging teeth to make sounds ("ooh"s and "ahh"s in the first month or two, babbling shortly thereafter). Soon those sounds will become real words ("mama" and "dada" may slip out and bring tears to your eyes as early as 4 to 5 months)." The site is The Baby Center, I like the information on the site and go there a lot.
http://www.babycenter.com/0_developmental-milestone-talki...
I know you begin to doubt yourself when you hear it at such a young age. My little girl is 7 months and at 7 months week one she said BOO! My husband still argues with me that "she is just making a sound and doesn't really know what it means so it can't be counted as one of her first words" BUT that is what words are sounds and every first word is just sounds, comprehension of that word comes later. My daughter LOVES peek-a-boo, I always say "BOO!" and she even says it mimicking my physical traits as if to scare me. But I started to think to myself - is it wishful hearing? Then yesterday I happened to be video taping her and I got 3 minutes of video gold! She crawled for the first time, sat up by herself (she just started to do that this week), clapped her hands, waved her hands in the air when I said 'YAY" and then realized I was looking at her and said "BOO!"
Gotta love the video proof! But even if you don't have a video camera. I am here to tell you I BELIEVE YOU!!! LOL Your child is just awesome thats all - giggle