Our DS is 21 mo old and still only says maybe 8 correctly pronounced words and hasn't added anything new to his vocabulary in several months. He is consistent though in making up words for things, or at least trying to say them correctly (like "dat" for light). He also knows a few sign language words.
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However, he understands much more and can follow many specific instructions (i.e. put your shoes in the closet).
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I read in the What to Expect the Toddler Years book that so long as they are progressing (ie. comprehension and following instructions) and seem normal in every other way, that some kids are just slow to talk and one day, the switch goes off and look out!
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Our DS was a 5 week early preemie and was hearing tested when he was days old and everything was normal. He has bascially lagged behind in most everything (motor skills, walking, etc), but eventually catches up and is normal in every way. We did check into Early Intervention, but he was deemed normal by the case worker that visited over the summer.
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He seems really smart and clever from what we observe about him, so we are not worried and his ped is not concerned.
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It's tough to not compare to other kids the same age, but all kids progress through stages at their own rate.