How often is a twin missed on ultrasound?

I went in for my first u/s today (was supposed to go in next tuesday) and they told me I was having 1. I was upset because I was so much bigger than last time, and I thought that I would be having twins, just to find out I was only fat. lol. so I get a call telling me that there are at LEAST 2, possibly 3 in there. I was like What the heck?!?