pregnancy update. (10 weeks)

So we had our first baby appointment on Monday. Well when I got into the room they told me I was far enough along (10weeks) to do and over the tummy ultrasound. Well as soon as she put the doodad on my tummy I thought I saw a membrane next to my baby. I asked the lady if there were two. She said, "wow you're really good at reading an sonogram aren't you?" I said, no and she continued still only one baby, I asked her, "are you sure, it looks as though theres a membrane there." She said, "hmm". At this point I wasn't sure if she was understanding and then we saw it. There's two babies! I was baffled, not knowing what to say. But we are super excited a few days later.... WE ARE HAVING TWINS!!!!