Linzy - I have a friend who went through something slightly similar...
She waited til around 20 weeks to see someone, or so she thought! While in there the doc did her fundal height & it measured at only 4 weeks to go!
I was so jelouse that she got half way through her pregnancy without realising, lol.

So, anyway, they send her off for an emergency u/s to find that there is still 4 months to go, just that there are 2 bubba's in there!