Hi Erica -
I understand your worry. But, honestly, I think you're doctor is a little behind the times. I know you'll get some wonderful information from the others on this board, but until then, here's some information to ease your mind a bit.
Here's a link to an article from the Physician and Sportsmedicine Online:
http://www.physsportsmed.com/issues/1999/08_99/artal.htm
This article is from 1999 - and it is my understanding that the guidelines have actually been updated again - to include a recommendation to exercise for 30 minutes "most days" during pregnancy. You may want to print a copy for your OB - however - some doctors are not too fond of patients bringing in internet information.
There is also an ACOG pamphlet (you can order it for free online
http://www.ACOG.com/from_home/pubqry.cfm) titled, "Exercise During Pregnancy". (do a search for "exercise, pregnancy" - it will show you the publication #AP119) I'm certain that your doctor might put more stock in this publication since it is the old ACOG guidelines that your doctor is quoting. You cannot read it online, so I'll quote you a few things:
First it states, "Becoming active and exercising on most, if not all, days of the week can benefit your health in many ways."
And then, in the Heart Rate section it states:
"The extra weight you are carrying will make your body work harder than before you were pregnant. exercise increases the flow of oxygen and blood to the muscles being worked and away from other parts of your body. So, it's important not to overdo it. Try to exercise moderately so you don't get tired quickly. If you are unable to talk normally while exercising, our actvity is too strenuous."
As for the miscarriage statistic, thats all it is, a statistic. You have to look at it as a 65% probability of everything being fine.
There are very few things that a woman can do to that will cause a miscarriage - mostly use of illegal drugs, etc. If you've been exercising at your normal level - I can't think of any reason for you to be worried that you've done something wrong.
Good luck to you!
Jennifer k