Hi, I was on Clexane from week 8 of pregnancy right through til 10 days post delivery.
A day can make a difference in clotting. The effects of the heparin drop off as time elapses from the injection. The 12 hour rule is correct, although I'm sure the rule would be bent in an emergency. As I understand it, the risk comes from the epidural rather than from blood-loss. There is a slightly increased chance of a spinal bleed when an epidural is administered when there is Clexane in your system.
I was so worried about this during my pregnancy that I spent a lot of time googling. Forget the actual figures now, but the risk is v v low.
In the event, I had a 13 hour labour before having an emergency section, so enough time had elapsed since my last injection.
With regards to transfusions, I had an abrupted placenta, caused by a low-lying placenta. Even with this blood loss, and a c-section, I didn't require a transfusion.
For anyone on Clexane not having an elective section, a top tip passed on to me from a Haematology nurse. Most women go into labour through the night and early hours of the morning. Therefore doing your injection late morning/lunchtime gives you the most leeway.
Camerashy - don't really have any experience of your circs, but surely if a clot is going to affect growth, then the final weeks are most important, with lots of foetal growth occurring then??
Hope this helps. For what it's worth, I recovered well post delivery and now have a gorgeous DD who is well worth all of those nasty tummy bruises! And I am planning on doing it all again asap!
Good luck with your pregnancies girls.