In most cases placentas only grow inside the uterus, however, I recently
saw a mother who had a four hour c-section (a surgery that usually takes about
45 minutes) because her placenta had grow through the wall of her uterus and
wrapped around both a ureter and the bladder.  This condition is known as
placenta increta (I'm not totally sure about the spelling).
     I don't know if when removing the uterus it would be possible for any
placenta to be left behind.  Our doctors painstakingly tried to remove it all.
And I don't know if any small fragments left might be enough to impair milk
supply or if they would be persistant due to the disruption of their blood
supply (due to the surgery), but I just wanted to mention it since the
question came up.

     Cindy Neary RN