Dear Possible (good name 'cause everything is possible),
For my case, when I knew I am pregnant, it was 2 months, and the doctor said they were twins after using ultra sound.
If you want to make sure, I suggest you to consult another doctor to double check. Because I also consulted 2 doctors at that time, the first doctor said I had twins, but the second doctor was not sure. But afterwards when I talked to the second doctor that I should have twins, then the second doctor check very carefully and finally sure I had twins.
So don't worry about that. At first, it's much better if you consult 2 doctors first for more safety.