Belgian chocolates... well, it's very interesting that throughout this whole thread so many women have written insightful, painful, articulate, HONEST posts about the kind of help that would be helpful, and the kind of help that is not only NOT helpful but is also actually pretty offensive. And yet you still managed to come up with that incredible story about your three SILs. Why is that, I wonder... .
As for stress playing a role in infertility, well, the thing is- there is stress PRIOR to finding out about infertility. Are we talking about that kind of stress? Because when you say it may play a role, you are referring, I assume, to the causative factors of infertility? About that, I don't know. For all I know, there may well be some cases (pretty few, I would think) where depression or anxiety is so severe that it may prevent ovulation. But you know, lack of ovulation is a relatively straightforward fertility problem to resolve. Plus, just to let you know (since, I'm sorry to say, you don't seem to know much about the topic) there are hundreds & hundreds of women going through horribly painful & stressful (physically & psychologically) fertility treatment, due e.g. to a sperm problem that their partner has, & still they have absolutely no problem with ovulation, plus they (surprise surprise) manage to conceive during IVF / ICSI, one of the most stressful treatments (as others have eloquently explained). So the kind of stress that temporarily may cause infertility is in my book pretty irrelevant & usually doesn't last long.
What is definitely the case, is that infertility causes stress. So, stress doesn't cause infertility, but infertility certainly causes stress. There is not one- NOT ONE- person going through infertility who doesn't at some point get stressed, depressed, whatever you want to call it. The anxiety, the not knowing, the seeing all your friends conceive at the drop of a hat, the constant contact with pregnant women & babies, the painful (and expensive) treatments, the rollercoaster of the endless 2-week-waits. The list goes on. So certainly all this causes immense stress, and UNDERSTANDABLY. You can imagine, then, that telling someone (supposedly trying to be nice) to 'relax' and 'not try so hard' or that your 3 SILs got pregnant once they stopped trying, is at the very least insensitive & the very worst mean & offensive. And by the way, for your information (and again, others have written about this). Infertile couples manage to get pregnant- with excellent success rates, increasing all the times- during exactly this, the most stressful period of their lives. What do you say about that? The success rates of infertile couples doing fertility treatment are hugely, hugely increased compared to their previous status, 'waiting', 'relaxing', 'taking their time'.
Final point. If we want to talk anecdotal evidence (which as you should know is no evidence at all). I know at least 5 people in real life who went through adoption (in 2 cases), numerous failed IVF cycles (in the other 3 cases). Guess what. They then decided to give up & NEVER got pregnant, 5 or 10 years down the line. And before you tell me 'there's still time', think again, they're now hitting menopause. But maybe there is a miracle menopause story too that you can throw at us.
Just to say: this is not a personal attack, lots of people say these things, believe me, they're not unusual at all. This is just a post written to make a point, with the hope that the point will get across...