What can cause blood clots in umbilical cord and dead spots on placenta?

First of all this is a very hard topic for us but we would like advice..My sister was 31 weeks and 3 days pregnant (May 4,2011) when she had noticed some decreased movements so she called her OB and they told her to go straight to the ER. She got there and they took her up to Women's Services and they hooked her up to a monitor but when they went to try to find the heartbeat they could not ..they seen a flash on the monitor but just a couple nothing steady..anyways they laid her on her back and tried and nothing. They called the doctor and he rushed right over and did the ultrasound and told my sister the WORST news anyone could hear...that her baby boy had no heart beat :( They do a c section and he was born at 11:44am weighing in at 2lbs 7 oz and 15 1/2 inches. The dr came in and told me and my mom that there were numerous blood clots on his umbilical cord and a good 2 or 3 big ones and that there were a few dead spots on the placenta so he said he was sending the placenta off to the lab to try to get some answers...well he has been no help to her at all. he said it could be her low iron or borderline anemic but she was taking meds for..then the dr that looked at her incision (which is awful looking and bruised from the top of the incision to about mid way to her belly on) said that sometimes the placenta can separate from the uterus and cause it or her having a clotting issue and people not know they have it until something like this happens..She has a 13 month old son that was born April7 2010 and he weighed 4lbs 9 oz and he was delivered 2 weeks early due to cord around his neck but healthy but however Bryson did not make it and was born stillborn. We laid him to rest yesterday:( so anyone that has any advice would be helpful thank you