I had a missed miscarriage about four weeks ago, and had a d and c two weeks ago to complete miscarriage. So that was about two weeks ago and i still have a preg. positive test. It takes about 3 weeks for HCG levels to go down enough to show neg. on a preg. test. So dont worry, but you might ought to go to doc to make sure that there is nothing left in your uterus. If there is it may have to be removed if it isnt coming out alone. You dont want to get an infection.
So sorry to hear about this! But dont worry, you will get that neg. preg test soon and will be able to start trying!