I just came in to post the same question! My DD is 20 months and has a couple of words and a couple of animals sounds. She seems to be developing very slowly to me. Her physical developments seems fine but her speech and comprehension seems behind. We read the same books all the time, point out the same pictures and she still can't point to any by herself. She is not doing anything that DS wasn't doing at 10-11 months. By this age DS had a vocab of around 500, speaking in sentences and could recite his books from memory. DD is miles away from that.
My SIL DS & DD were just like this. Her DS now 3.5yrs, talked really early. At 18 months he new how to count to 20 in 3 languages, the ABC could say everything really clearly. Her DD now 21months is only just starting to talk. SIL was worried because of DS talking so early. But like PP say they develope at different stages and I think they can only work on one develpoment at a time sort of thing.SIL DS did not walk till 14months, DD walked at 10months, so things that DS took a long time to do, DD had them down pat early. If all that makes sense.