Chances of losing pregnancy at 18 weeks?

I'm 23, perfectly healthy and the pregnancy has been too. But 2 years ago another OB diagnosed me with something that he claimed wouldn't let me get pregnant or cause me to miscarry. I went back to my original OB and they said I don't have that and everything is perfectly fine. Should I still be worried? What's the chances of me losing this baby? I'm 2 days away from 18 weeks.