How To Find The Cause For The Miscarriage?

i just have had an abortion of 20th weeks of pregnancy. it was very sudden even doctors could not find out the reason and they coudl not save my baby. i want to know what can be the causes of my miscarriage? bcz on 25th of may in the night i was addmited in the hospital bcz of spotting and normal leakege and in the morning of 28th i felt pains and after few hours i lost my baby. I m very dipressed bcz this thing has happened me twice? plz suggest me what to do?