I was 5 weeks pregnant when I was told I was having twins.
During my first visit to my OBGYN she did what's called an internal ultrasound and I even have those funky print outs or pics from it.
What you see is basically two circles on the screen which is indicative of two sacs or two babies. (My twins were girls but NOT identical btw)
She told me that one sac was smaller than the other and may not make it. There is something called a Vanishing Twin Syndrome (swear to God check it out if you don't believe me:
http://multiples.about.com/cs/medicalissues/a/vanishingtwin.htm )
She said alot of pregnancies start out as multiple pregnancies
without the woman even being aware of it.
Because one of mine was smaller to start with, she was concerned that would happen with me. I went back a week later to see how the second one was doing (imagine for a week wondering if you would have twins or not!!!).
Well, in that week's time the second one grew a surprising amount and today I have healthy twin girls as a result of that pregnancy.
So, I just had to comment on those who say it's not possible to tell so early on that you are having twins. I knew at 5 weeks.
Now as for everything else that's being said, I am staying out of it!!
I will say that I didn't learn the sex of my twins until a regular old ultrasound at about 6 months.
Carry on......