Well, here is my purely anecdotal story.
Prior to DC1, I had no issues with my period. I had regular, "typical" cycles.
Post partum, my periods were full of days of spotting. Some months I felt like I was bleeding the whole time . . .spotting, and then my period. After reading TCOYF, I knew I had low progesterone. Thankfully, I was successful in getting and maintaining the PG with hormones.
My doctor told me it was due to age (I was 31). I thought it was due to nursing-- it was the only thing different. I expected it to be a life-long issue, if only just annoying when not TTC.
When my period came back after DC2, I had NO spotting, and she was nursing constantly. Our first cycle where we timed things right, I got pregnant.
So, it wasn't age, it wasn't nursing. What was it?
I can only assume it was stress. I was under so much stress during that period of my life (postpartum with DC1) for various reasons. I think my body was not letting me have more stress by getting pregnant.
I was annoyed with people who told me that I couldn't get PG because I was tense. I thought that was a myth-- I knew it could delay ovulation and in extreme cases for long periods of time, but I refused to believe I could actually cause myself to not get pregnant by being stressed.
Now, of course, I don't think most serious TTC issues can be traced to a simple matter of stress. However, in my case, I am now almost sure that it was.
2/02, 4/05, 2/07, 11/09, and EDD 12/25/11