With my girls I never lost anything until the last moments of birth. dd1 it was within the last half hour of birth and dd2 it had to have been in the last 5 mins.
I didn't really have increased discharge, I never had any blood, no lumps of mucous, no waters breaking etc. dd1 I was in the water and felt a pop in the last 30 mins and dd2 I was wearing undies until the last 5 mins of birth and then I jumped in the pool so I know I hadn't lost anything until at least that point. My undies were totally clean and even after weeks of prelabour I never lost anything.
So you never know, you may just go straight into labour!