What Causes A Miscarriage?

Hi it s me faiza. I am 28 years old. I had been married for 6 and a half years. I had a daughter of age 5 and a half years. After years of her birth a spontaneous miscarriage occurs which was of 7 weeks. Later on after a year again I conceived which leds to abortion in 5th month (anencephalic).Now again a miscarriage which was of 9.6 weeks as it didn t cleared out with medication( no cardiac activity was recorded). After taking pains for consecutive 7 days . MVA was done by the doctor. And later on detailed pelvic pathalogy was done which revealed that uterus is cleared completely but contains a streak of fluid found in cul de sac Kindly need your recommendations