How do i know when my labour has started ?

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or is it obvious?

It's obvious. Believe me, you'll know.

It isn't necessarily obvious, but it will be when you're having quite noticeable contractions. I actually had painful Braxton Hicks throughout my first pregnancy, and they just mounted for my first labour..and as I'd been convinced that 'you just know' when you're in labour I thought I couldn't be in labour at all. I was wrong, as I realised when they built, and built, and BUILT...

mine started with stomach cramps, like period pains, but with more ooomph! Then waters broke. Then more cramping, harder and breath-taking-away..and so on

You will know...

cos i've ust been to the loo and think it was slightly brown like bloody an dhave been having BH but not sure have put a panty liner on and will check - oh yes am also trying to pack my bag !

once you're in established labour, you're sure to have noticed, until then it might even be nicer to not know and jsut think they were cramps/braxton hicks.
the day my labour started, I woke up, puked everywhere and felt like I had food poisoning. duh! contractions didn't start til the evening though but looking back, that was a sign

ooohh i thought i had food poisoning yesterday as it was coming out both ends in the afternoon and last night but felt better this morning¬!!.

Agree with MI. There's a far few early signals (bottom clear outs, having a show etc) but you'll only really know once the contractions start hotting up so much that they hurt alot. I had niggly period pains, which I thought was just the baby's head settling further down, but they got worse and worse and worse until I was struggling to talk, and then thought "Oh perhaps these are contractions" Duh! (no show, no braxton hicks, no diarrhoea). Baby arrived 5 hours later.

How are you doing?

You'll know when you're trying to get to Tesco but can't get off the loo! Or you'll have suspicions, at least. Once you go past a certain level of pain, it is obvious. Before that, you might not want to think it's labour because of disappointment if it's a false alarm.

okay am suspicious will let you all know - watc this space

I remember zoning (leaving the planet mentally) when mine started, although I was induced 3 times in the 72 hours leading up to it (which didn't work) and I can remember a burning sensation everytime I had a contraction all over my lower abdomen.