What Causes Repeated Miscarriages In A Period Of 9 Months?

Hi Doctor,2 times miscarriage within the difference of 9 months.FIrst time (April 2014) i got the brown discharge started just after getting the pregnancy test positive. Doctor prescribed me vaginal progesterone and folic acid and asked to wait for next ultrasound report. After 2 weeks scan resulted in empty irregular sac. It was the spontaneous miscarriage.This time Feb 2015 again i conceived and got the test positive. but from next day itself again i got brown discharge started. Same day i went for scan and found that till that day everything was alright. Regular sac was there with a tiny yolk corresponding to less than 5 weeks. Next day i got some spotting as well along with brown discharge. Doctor started duphaston and asked to wait till next scan. After two weeks scan, no fetal pole was seen after 6 weeks 5 days pregnancy. The yolk was still tiny. Doctor said its not a good pregnancy and should not be continued. Hence she cleaned by some tablets.I am worrying about my next pregnancy. Do I have any problem in my uterus or any deficiency of hormones? What was the cause of these types of miscarriages with similar symptoms.Please suggest me something as i am seeking for guidance from expert doctors.