What Causes Miscarriages?

hi doctor , i take femilon tablets to last -5 years. i have a 6 years old boy. last 2 years i have 2 miscarriages. one is not developed and i am missing that within 3-4 months ,i got periods. another is within 1 month i got periods. sir what is the reason for it i am going to doctor but i have no satisfied answer. please tell me what can i do.now i am 28 years old