What Causes Miscarriage During Third Trimester Of Pregnancy?

Hi, I had a miscarriage on the 7th week of pregnancy. it was my first pregnancy. I took CMV, Ureoplasma ssp., chlamydia trachomatis test and there were no problems and viruses. I have had no serious health problems and my family members too. What else do You think I should check, what other possible reasons there may be? Thanks in advance.