this is interesting because my work pal & I are discussing this very thing.
I was a mistake. So far I've never managed to find out the real story of what went on.
There are various ways I just knew. When my mum got divorced, I was 18 & she would tell people, in my company, of 18 years marriage gone.
I visited the census place (with a friend tracing her family tree) & decided I would look up my parents marriage date. I found it a year after they 'should' have married. I asked my mum the direct question, asking her if she could please tell the truth & of course she lied.
Having spoken to my gran about it, it was obviously a tramatic time all around. My parents never had a happy marriage (so my childhood was full of rows/silences/atmospheres etc) & I frequently wished I wasn't born - also an only child.
I think the key here is to be honest - if my parents had been in love & been happy, it wouldn't matter 2 hoots.
I also think unplanned is a better way of putting it. Life's never perfect, but trying to cover up, just makes everything worse.