In pregnancy your ribs are stretched -- expanded -- beyond their original position. This makes them more vulnerable to injury. They can even hurt and make breathing a little painful (like a stitch in your side) until they settle into a new position. You can crack a rib or bruise one by twisting wrong, coughing violently, and probably if you are term and your uterus has reach capacity which puts a lot of pressure on the ribcage.
Other than that, random kicks won't hurt you because the uterus is a big muscle and the fetus is cushioned by amniotic fluid. It would take an incredible amount of force to be able to kick hard enough to do damage.
As for the kidney thing -- the kidneys are near your spine in front of some low ribs, I'd say injury was more due to organ shifting and unusual pressure in the same spot than the fetus because the kidneys would be behind all the other squashed organs.
at 9:27 AM on Jul. 16, 2009