What Can Cause Miscarriage?

HELLO MY WIFE IS 27 YRS, ACCORDING TO ULTRASOUND FETAL MATURITY IS 6 WEEKS 2 DAYS FETAL HEART ACTIVITY IS NORMAL FHR-128,CHORNIC REACTION IS NORMAL.MY DOCTOR PRESCRIBES HER DUPHASTON 10 MG TWICE FOR 2 WEEKS WITH FOLIC ACID TABLETS ONCE AND INJECTION HUCOG 5000IU WEEKLY.ALL BLOOD AND TORCH REPORTS ARE NORMAL.IS THERE IS ANY CHANCE OF MISCARRIAGE& HOW MUCH HEART BEAT IS NORMAL ? IT IS FIRST TIME PREGNANCY.