Before relating how the wife found out, I have to say there are very few times in my life I have been truely, jaw-dropped surprised. The last time I can remember was my 19th surprise birthday party (the two are not really comparable).
I had completely written the possibility of twins out of my mind as of the first meeting with the doctor. My first meeting with the doc was the wife's 3rd meeting, and I naively thought that any big surprises would have been out by now. Our ultrasound was at 18+ weeks, which is half way through. As a commenter mentioned in this post, there were surprises like this in the past. I figured having two show up at birth instead of one was so far in the past that I would have known the size of the team sooner than at the half way mark.
An interesting aside, less than a week before we found out we were having twins we were at our first baby sale. Picture a packed community hall, 30-50 women, each with a table covered with baby clothes and toys. I hate shopping unless it's power tools or some sort of electronic toy. So as the wife is looking at baby clothes, I wandered into the larger item room (strollers are sort of like a machine). The wife wanders in after me and we see a double stroller there.
I remark to her, "At least we are not having two". Her reply:
"I was just thinking the same thing".
Part Two tomorrow