We had a full pm done on our angel, I had to. I knew we might not get answers, but I had to try, I couldn't cope not knowing why this had happened. I needed to know and if there was a chance I'd get some answers, I had to take it. If I remember right, it took about 8 - 10 weeks for everything to come back, and we got half answers.
Our angel died from a circumvellate placenta and hypercoiled umbilical cord, which also caused IUGR. At 20 weeks when he was delivered he had been dead since 17 weeks, but was only measuring 13+5. We didn't get a reason why that had happened, but it still helped to know there had been a reason, even if there was no reason for the reason.