More newbie questions 
I watched my first (and last) episode of One Born Every Minute the other day and a girl was given an injection to help her 'deliver the placenta', I assumed it came out with the baby, I've seen a few animals give birth and after the young has been born the placenta just follows.
Is it different for people? why do we need injections? do we have more contractions to do it? is it straight after birth or can it take a while? is it big? and finally where does it go, just all over the bed?