My DD was not prem and was not talking that much at that age, and she is an intelligent, normal kid. I keep getting worried when i compare her to a couple of other kids, but then when we have her assessments and she is in the normal range. By 18 months, speaking 5 words is normal. By 2 years, they like kids to have 20 - 50 words. When i stop comparing and just focus on DD, i see all the cool things she does and how many different things she learns every day.

ETA -my niece had less breastmilk and more formula than DD and her language is alot more advanced than DD. i really don't think that the amount of breastmilk affects speech at all.