18 month old DS babbles but doesn't talk. He says "mumumum" and "dadadadad" in a babbling way, not consistently, has a babbling noise for when he wants milk (along the lines of "blooblooble"...) and says "car" (his favourite toy) but that's it. He said "duck" on and off for about a week a few months ago then stopped 
His understanding seems good, he can "put it back" etc. But I see friends' children his age requesting things by saying "please, mummy" and its kind of breaking my heart that he's not communicating yet. I know "they all get there in their own time", but when should I worry? I don't want to be over-anxious but I don't want to be lax either :( everything I've read says they should have about 20 words by now and really we only have 1! :(