Fetal chromosomes. The most common cause of a miscarriage is an abnormality in fetal chromosomes (meaning the fetus had defects such as gene mutations). There are other rarer causes, such as low levels of the hormone progesterone, problems with blood clotting, or an abnormal uterus. I am very sorry for your loss.
Hmmm. I am so sorry for your loss; but miscarriages are the bodies way of telling us that there was something wrong; and ultimately your body handled it. Unfortunately, i cannot give you a more detailed answer(something with the fetus at the genetic level; some unknown anomaly...Or perhaps an undiagnosed clotting disorder?) but your doctors can; please discuss this with them and do not give up hope!