The problem with coffee, or strong tea and cola drinks for that matter, is that it is a diuretic and basically make you go to the loo more often, so you pass more fluid instead of the body getting the benefit. Also it can reduce the amount of iron your body absorbs from food and that can be a problem, particularly for pregnant women or if you are anaemic. So you should make sure that you drink pleny of non-caffeinated drinks during the day in addition to the coffee.