I have an 18m DS. In public he will not utter a word. At home he can go a few days between saying one of the two words he can say (bubble and ball)
His understanding is fine - you can say to him "go into the playroom and fetch a book and we'll read it" and he trots off, goes to the bookshelf, grabs a book and brings it over. But EVERYTHING is "da". He wants some water, he brings his cup over and says "da". He points at a dog, or a spoon, or a bowl of food and its "da". Da also doubles as "whats that?" He likes to point at things with an inflection in his voice so it's a question "daaa????" and I say "thats a rabbit/thats a baby/thats the television". He can keep that up until my blood pressure is rocketing. "THATS STILL A RABBIT" (he asks "da??" sometimes over and over again pointing at different things)
He makes no effort to speak though. If I say "can you say water?" he'll just look at me blankly and then get really frustrated if I don't produce the water, so its no good waiting and making him say it. He just plays dumb if you ask him to say a word.
We have no TV on in the day, speak to him loads, read books, he's still rear facing in the pushchair so I can still chat to him when we're out.
Every month we say oh he'll start talking soon but there are babies younger than him at toddler group now (12/13 months) really overtaking him! When should we be 'worried' or ask for a referral?