Chances of a stillbirth occuring again?

My daughter was stillborn 3 weeks ago at 31 weeks. I was having a healthy pregancy so this was unexpected. All the ultrasounds they did earlier on showed no problems and all the blood work I had done came back normal. No history of high blood pressure or diabetes. I delivered her and the cord was wrapped twice around her body and once around her neck. The placenta and cord were perfectly normal and she was measuring 31 weeks. I did not have an autopsy done b/c I could not think about them cutting on her. The blood work from her and me all came back clean from infection. What could have caused this? Can a cord really strangle a baby in utero? What are the chances of something like this happening again b/c I will always worry?