MrsMc - I was sort of in your position. Was taking clexane through pg but not aspirin. Like you, was advised not to do the injection if I thought I was in labour.
They advised me to inject first thing in the morning. Apparently, most labours begin overnight, therefore you have more chance of being clexane-free for your labour.
For me, I did indeed go into labour at 4am, therefore was about 20hrs since my last injection, well over the 12hr limit.
I'm perhaps the wrong person to advise about 3rd stage labour though.
I was in labour for 13hrs, and ended up needing a section. This was due to placental abruption (unrelated to clexane use btw, due to low-lying placenta). I was able to have an epidural, as enough time had elapsed since my last heparin.
Post delivery, they doubled my clexane dose for about 10days, and was put back onto warfarin.
Breastfed successfully for 6months, until DD grew teeth
I don't have a recognised clotting disorder, but had a DVT and PE pre-preg. They took me off the warfarin at 6months post-natal, and I had another DVT. Am now on warfarin for life.
Am currently waiting for an appt with my consultant with a view to going onto heparin again so I can ttc (warfarin causes birth defects). So although the jabs caused me some discomfort (and worry at the time), I feel confident in doing the whole thing again!
Try not to worry about your labour. They will be well aware of your increased risk factors, and will treat you like a queen
HTH, and good luck