Hi there - I'm a birth doula...Here's what I pack in a small, black rolling suitcase:
Birth ball w/ cloth cover (obviously not in suitcase haha)
massage items (wooden roller, hairbrush, stress ball thingies)
Thick towel (used as a knee pad)
iPod with external speakers (with preloaded lists of birth music)
change of warm clothes
toiletries for me (toothbrush/paste namely, plus hairtyes & tampons/pads)
plastic grocery bags and ziplock bags (myriad of uses, usually for changed clothes)
snacks/bottles of water for me
packets of EmergenC for me
Rice socks (one per client, so I have several in the suitcase)
Big bottle of antibacterial gel (yay, hospital germs)
Gum/mints (gum for me, mints for mom since we're upclose&personal)
change for vending machine/parking
colorful card stock with permanent marker ("natural birth in progress, I will ask for pain meds, please don't offer" & "I'm breastfed, please no artificial nipples!")
Back-up doula's info
client's birth plan
camera
notebook & pen
Candy for ornery nurses
Aromatherapy oils: I find spearmint most helpful for moms
My business cards
This seems like a lot of stuff, but my fairly small suitcase is nowhere near to being full. I'm looking forward to seeing what others include in their own birth bags!

