They don't work this out via some kind of official registration, as has been said, that would be impossible. This is merely a mathematical model which maps global population and predicts the population at any one time in the future.
Given this, I can't see the global population continuing to rise for much longer. Some people think we will reach peak population by as soon as 2040. I'm of the opinion that we wont go over 9 billion. This boom in population is due to the rest of the world industrialising itself in places like India. Once they become better developed, then education and medical standards will increase. Many countries in the world already have a decelerating rate of growth. We ourselves are not far from this.
It's not a perpetual thing and it wont go on forever, it's just a matter of 'when' it will slow and eventually start to drop and 'if' we have the resources to cope until then.