Twins

Okay, ive just assumed there's only one baby in me. And I'm still very convinced there's only one. We've heard the heart beat twice at the doctors but won't have an ultra sound until 21 weeks.. April 7th. My fiancee is convinced it's twins because of a weird noise we heard in our home doppler. Just sounded like two heart beats. But I don't think it was two. Anyways. My question is, since the doctors stop looking for a heart beat as soon as they find one, is it possible to have two in there? And just not know yet because the doc wasn't looking for two. I hope this makes sense.