I had to start taking iron on top of my Multivitamins at around 36w with DD in 2010 as I was very low in iron after a blood test. It was a waste of time for me, she was born 4 days later, so I can't really comment on their effectiveness! I resumed using them a couple of weeks after her birth too, I wasn't coping and recovering well. They did help slightly, not enough for me to rave about though.
I was just given a piece of paper by my middie after a routine antenatal visit, to take to the pharmacist. All it said was: low Fe. I got a small container of pills and that was that. They cost me $5.