So, um, at what point in a water birth does stuff appear that requires sieving? 
I'm planning to use a pool for pain relief, and my current plan is to get out for the actual birth, but I'm leaving open the option to stay in if I'm happy there. Depending on when the baby comes, one of my birth partners will be a friend who is on the squeamish side, and I'd like to be able to warn her when "ickiness" might appear (she's said she's going to work on preparing herself to handle things, but the more info she gets, I suspect, the better off she'll be).