At what point/age did you pursue an evaluation for your child's speech? What prompted you to do so?

My daughter's pediatrician at her 15 month visit said she was not concerned, and they only "need" 4 words at that point to be on target. She's 17 months now and has several words and even phrases, but there are some sounds she really struggles with that I feel like she shouldn't be at this point (/m/, for instance).

I'm not doing anything until after her 18 month visit regardless, but just curious!