I didn't get a chance to read all the previous posts in this thread, so please excuse if this is redundant...
a calcified placenta indicates that the placenta is getting old, and is not doing its job well anymore. The additional indicator of low amniotic fluid corroborates this. If your placenta isn't working well anymore it isn't getting as much blood to your baby. This causes the baby to pee less, and hence, less amniotic fluid. You can physically feel the calcium crystals in the placenta after the birth - it feels "crunchy". My daughter was born 9 days past her EDD and they were going to induce because of low fluids (but didn't have to - I was already in early labor) - after the birth our family practice doc talked about calcification of the placenta and actually handed my husband a pair of gloves so that he could feel the calcium crystals in the placenta (mine was in ok shape - just starting to calcify).
You don't mention whether your baby was born long after your EDD, but this is the main reason that happens. Not much you can do about it I don't think.
Mom to James ( 5/2006), Claire ( 6/2008), furry kitties Calvin and Bob, and wife to Dennis.