Population has become a dirty word. It is a word that many of my predecessors and counterparts have, some might say understandably, steered clear of for decades. That's because it is normally followed by words like 'control' and 'explosion'. It conjures images of forced euthanasia and sterilisation at one extreme or famine, poverty, disease and war at the other. But as the world's population reaches seven billion - with the number of babies born each month equal to the population of Portugal - now is the time to start talking about it.