Ultrasound. Ultrasound. Ultrasound.
Many women think (wish?) they were having multiples. I'm not sure why, but this is a common thought among pregnant women. If you want, I can dig around for some statistics on this - but the only for surely positive surely sure way to know is to have an ultrasound (or wait and give birth, which is how they did it in the 'old' days).
Good luck.