Should my 18 month old be more advanced?

I have an 18 daughter and she is our only child. I find she doesn't talk much out all she can say momma and dada and that's it. She will say the sounds of words like please or thank you or stop but it isn't the actual word. She doesn't know any letters numbers or where her eyes ears etc is (even though we have tried teaching her many times). Since she was born she has had extremely good hearing and sight. My mother works with a speech therapist and asked about this and she said she was behind. But when I mentioned it to my husband and mother in law they freaked out and fussed me for even thinking that. I guess they don't want to admit that it is possible for something to be wrong with their daughter/granddaughter. Anyway what do y'all think? Should she be more advanced than what she is? There is a poster in her doctor's office that says how many words she should say and such and she is definitely behind going by that, but I know every situation is different.