The average time to ovulation following a miscarriage is 6 to 8 weeks and most women have resumed their menstrual flow by 3 months following miscarriage. I ask patients to use barrier contraception from the time following miscarriage that they resume relations until they have a normal menstrual period.

The hCG levels linger following a miscarriage and may give you a "false positive" pregnancy test. It can take 6 weeks for these levels to fall to undetectable amounts. Thus, if you think you are pregnant, have a blood test done by your physician and a second quantitated level after 3 days. The pattern between these two tests will determine if this is a new pregnancy or residual hormone from the old pregnancy.

Credit: http://www.medhelp.org/forums/maternal/archive/435.html