Hello Doctor, I Had My FET IVF Done On 29th

Hello doctor, I had my FET IVF done on 29th July 2016 with 5 days embryo transfer( first IVF was not a success) After 12 days my hug level was 75 which my doctor confirmed as positive preganancy. Then after 15 days I was called for routine scan and they did TVS and told us that the heartbeat was irregular but need not worry. They gave some medicine and injections and asked us to visit after 15 days. The same day I got my ultrasound done and the doctor said that the heartbeat is normal and it was like 145 beats and also cardiac activity was seen and he confirmed pregnancy of 6weeks 4 days but told that the growth is one week slow. Now yesterday we again went for scan and the doctor did TVS and ultrasound and told that there is no cardiac activity and it is a miscarriage. Growth was like 8 weeks and 2 days. How it is possible when I was taking all medicines on time and have a healthy lifestyle. What could be the reason? Is there any chance that the heartbeat can restart? Should I wait before going for abortion. Please advise.