How would we know if there's multiples w/ no US

Okay, so I'm in the US and I'm not expecting an ultrasound until 20 weeks or so. I've heard the baby's heartbeat once with a Fetal Doppler at 11w5d but it was pretty easy to find, no searching around for it. My question is, if there were two in there, how would we know if there hasn't been an ultrasound?? Would I find out about twins at 20 weeks? My tummy is growing pretty fast for 12 weeks and it's my first...unless the doctor magically knows there's one 0__o