I don't think anyone can give you an answer to your question. I know this must be agonizing (my sister has had 2 m/c and is now in her 40's). I'm so sorry that you are left with very difficult questions, many of us experience losses that leave us with many unanswered questions, I pray you are comferted and you find the strength to endure whatever lies ahead...Good luck to you!
Miscarriage is a very NORMAL PART OF REPRODUCTIVE HEALTH. Most women who have been pregnant 2 or 3 times, have experienced a miscarriage. So many people have a story. I am so sorry to hear about your miscarriage, I am going through it as well...I am 33, and have 2 healthy children.
I had an ultrasound at 6 and a half weeks with a normal heartbeat and perfect development. The next week, the baby had no heartbeat, and had actually had gotten 1 mm smaller. The baby had passed away, and my body had actually begun to absorb some of it already. The PRIMARY cause of miscarriage in the first trimester is chromosomal. We have 23 pairs of chromosomes, and what happens is that one of the 'pairs' will actually develop 3 instead of 2. As a result, it is not normal, and the baby just stops developing. The good thing is that when this happens once, your chances of it happening again are smaller than they were to begin with. Almost like the body doesn't repeat the same 'mistake'.
I don't know much about an enlarged placenta. Could it have been a 'subchorionic hemmorage'? This is when there is blood behind the placenta near the uterine wall and at any moment can cause the placenta to detach from the wall and cause immediate death to the fetus. Frequently, it does not detach, and a normal, healthy baby is born.
It sounds like you will be getting an autopsy on the fetal tissue, and you will have an idea of 'what happened'. I think it takes a long time to get the results. I didn't have a d & c, so it was not an option for me.
It seems like getting some scientific answers will definitely benefit you, and you can move on....this is something that is so difficult to go through...I still feel like I can't believe that it happend to me! But, what doesn't kill us, makes us stronger...