I was 12 weeks pregnant, my first kid. I ended up having (I think they called it a spontaneous miscarriage.) a few nights ago along with a D&C. I heard the heart beat about 20 minutes before actually having the miscarriage.. I was having the contractions and everything.

My question is, what's the chance it'll happen again? The doctor's didn't tell me anything was wrong with my blood.. just that my cervix was weak (again if I remember correctly) I also have a doctor's appointment the 10th of this month so I'll know for sure then, but at the moment it's bugging me.

First off; sorry for your loss.

Many pregnancies end in miscarriage before week 20: 10-25%... in fact many women get pregnant and don't even realize they are having a miscarriage but mistake it for a period. I wouldn't worry about one miscarriage too much unless there was severe bleeding or the tests the doctors ran came back negative. (So wait to see what the doctors say before you worry!! )

But, that being said a miscarriage can be due to genetic factors that are not assessed by most doctors after the first miscarriage (they usually wait until you have at least two... which signifies hey something else might be going on here). So if you're really worried I would check your family history as well as your significant others to see if there is a history of miscarriages or any type of genetic disease. If something seems out of place you can always see a geneticist who can help you figure out what might be going wrong.
Your age may also come into question women over 30 are much more likely to have miscarriages.

The American Pregnancy Association has some really good info about miscarriages that you might find interesting: http://www.americanpregnancy.org/pregnancycomplications/miscarriage.html

Good luck!!