I found out at a scan when I was 10 weeks I think. I'd had bleeding, which I didn't have in any of my other pregnancies so was freaking out thinking I was losing my baby. I had also been tired but no sickness so was convinced I was miscarrying. The lady doing the scan initially thought there were three- thank goodness she was wrong!
My maternal Aunty has fraternal twins so I'm guessing women in my family must drop a couple of eggs. My husband's brothers are identical twins, and one of them has identical twins, but this apparently has no bearing. Our twins are fraternal.
I was really worried and upset when I found out, but it's been really good so far and I can't imagine not having them both now! (they are two weeks old!)