Great day for the curious! UN releases World Population Prospects 2010 http://esa.un.org/unpd/wpp/index.htm
No more grouping of countries into developed and developing! Instead countries are grouped based on fertility into low (ex. Vietnam, Iran & Brazil), intermediate (ex Egypt, USA, Bangladesh, South Africa), and high fertility (ex Pakistan, Nigeria, Congo & Bolivia) !
The high-fertility countries (> 3 kids per women) are mainly in Africa and only have 18% of the world population = 1.2 billion people. They are expected to tripple their population to 3,6 billion this century! The Low fertility countries are 40% of world population and will decrease their population and the intermediate are 40% will level of at 2050.
The peak size of the world population, therefore, depends on how fast peace, school, basic health care, poverty alleviation and family planning will become accessible to all in Africa and a few other countries like Yemen, Afghanistan and Pakistan.