I have heard of this. I have a relative, and I've heard of others, who tried so hard to conceive and couldn't for year, adopted, and a couple months later did conceive. It's as though the stress was off and it happened.
I firmly believe that stress can prevent pregnancy. My husband and I went some highly stressful situations last summer, between him getting cancer, our money troubles and in general stressed and worried I wouldn't get pregnant. As soon as things settled down, I was pregnant! I didn't think so much about it, I had given up saying the attitude, as it's not my time but god will decide when the right time is. good luck to you!