I thought my DS who's 15mths was a genius doing ok with his talking. He has four clear words understandable by everyone Mummmmmmmmmmmummmmummm, Dad, cat & Up.
So apart from the 3 words he talks sentences of babble to himself when playing or exploring. It's a bit like those kids Tv characters that don't speak word just make noises "hmmmm hppppp hmmm cooooo munmnnnum ah" etc
But I keep meeting children his age and younger (particularly girls) with loads of unusual words. Had a just turned one year say "Bubble," "plane" and "Bee" during an activity class today, when pointing at said items.
I know some parents say little Johnny knew 50 word including hippopotamus at 15 months and I usually think oh yeah, but when I'm hearing babies the same age say "truck" "Digger" "bunny" I am thinking my DS is behind ?