According to speculation and study, what will happen to the global birth rate in about 40 years?

birth rate will increase
birth rate will decrease
birth rate will stay the same

The correct answer is - Birth rate will decrease.

In the past few decades, the birthrate, on a global scale, is decreasing. This doesn't mean that the population is shrinking though, on contrary, it is growing, but the tempo of growing is slowing down. Also, there's variations from one region to another, and while most of the world is experiencing significantly lower birthrates than few decades ago, Africa still has a very high one, with only very slightly reduction of it. Still, according to the prognosis for the next decades, the birthrate, on a global scale will continue to lower down little by little.