I've just finished a conversation with the umpteenth person to ask me whether my pregnancy was planned and it's made me wonder why so many people think that this question is acceptable.  
What really baffles me is that, once someone is pregnant and clearly planning to keep the baby, how can the answer to that question possibly be relevant to anything.  So it was planned... well that's nice.  So it wasn't planned... well, oh dear, but that's nice anyway.  If someone is willing to volunteer that they're delighted as they've been trying for ages, or that it was a pleasant "surprise", then fine, but why feel the need to ask? 
I asked my husband about this and he offered two solutions.  One was that men don't really know what sort of questions are appropriate to ask.  Well, I have been asked it by more women than men, so that blows that theory out of the water.  The other is that it is my age (thanks, darling!)  Apparently at the wrong side of 35 (I've just turned 39) and with a 6 year old child already, unless I've been unlucky, it would be an unusual choice to have another child now.  To that I would say that, if that is the rationale, then perhaps people should assume that the person has been unlucky and, if they haven't told you any of the details of their "bad luck", then they clearly don't want to talk about it with you.  But anyway, it might not be such an unusual choice - whilst most people aim for a smaller age gap, there are some benefits to having a bigger age gap and perhaps for that person those benefits are significant.
So I've concluded that it must be symptomatic of the age we live in.  With some people sharing every detail of their lives on social media sites such as Twitter and Facebook, perhaps we feel that we can freely ask a number of personal questions?  Well, it's not the way I roll, but maybe I'm just different? I'm off to see what Debretts Guide to Modern Manners has to say on the subject...