I would think if you were having twins, and you have already had any ultrasounds, they would have detected it then.
I am jealous about only gaining weight in the belly. I get very sick, but I eat to make myself not so nauseous. Then, I end up gaining a bunch of weight...although, I have lost 30 pounds so far!
I would like twins one day (not for many years), but my first ob said that with how sick I get with one, she suspected that my body and my mind wouldn't be able to handle a twin pregnancy. She said in her opinion, I would be just too sick. BUT, I have a friend that just had twins. It was her 2nd pregnancy. She felt worse with her first than she did with her twins...just really uncomfortable at the end of her pregnancy, and less nausea than her first pregnancy. So, who knows?