How far along are you? I would just check into seeing how much the ultrasounds cost and outweigh the risks and benefits to having or not having one. I think it would be worth the extra money to get one because you will need to be monitored a little closer if it's more than one (which that's what it sounds like). Congratulations and good luck.
I would wait until the 20-week anatomy scan to be definite! Unless you aren't wanting to find out the genders you may be able to do it as early as 14 weeks just to verify.
Some pregnancy clinics (crisis pregnancy centers; look in phonebook under Abortions: Alternatives) have sonogram machines and they'll give you a sonogram for free. It is usually NOT diagnostic but just allows you to see the baby (or babies)!