My son's language development tends to follow a pattern. Just as my wife and I begin to worry that he's lagging too far behind his peers and are close to panicking, he exhibits a dramatic increase in both producing and understanding language. It happened this summer and again just a couple weeks ago. We still want to set up a relationship with a professional, but it has lost its immediate urgency.
Now I know that language ability is not an explicit indicator of intelligence, especially at this age, but sometimes it can be hard to remember. I never feel that my son has regressed, something that might indicate autism. But I have sometimes worried that his lessened interest in numbers and letters--two topics he couldn't get enough of before he was 2--indicates that he's not quite the brain I'd expected. But I try to remind myself that, in his view of the world, his current enthusiam for construction machines is no different than his other areas of study.
When intellectualizing doesn't do the trick for me, however, little reminders of that sharp mind help. Like the other night when my wife was trying to get him to eat some apple slices. He wanted a pumpkin cookie, and she told him he could have it if he ate two more pieces of apple. So he picked up a slice, looked her in the eye with a devilish grin on his face, and broke the slice into two.
(He did end up eating two whole slices of apple upon our insistence, but he quite obviously knew exactly what he had done.)