Possibly twins?!

Today was my first appointment. Heard heartbeat on doppler but didn't get ultrasound. Dr says my uterus is really large (15-16wks) and I'm only 12 weeks even. (I'm sure of my dates, I know my ovulation date). Dr suspects twins. But this is also my 5th baby. Thoughts? It usually doesn't cross my mind it could be two, but this time I've had a gut feeling since I find out.