THe mother and child DO NOT share the same blood. (How would we have different blood types then?!) The Placenta acts as a filter, keeping the mother's blood out but allowing nutrients and other things the baby needs in. Some substances that can be harmful to the baby can pass through this barrier. These are things like alcohol (fetal alcohol syndrome), nicotine (low birth weight), and caffeine is also another substance that passes through to the child but it's effects are undetermined.
Everything passes through the placenta to the fetus b/c the mother and fetus share the same blood.