Most couples in western countries nowadays tend to choose not to have children. It is better considering that this way you can focus more on your relationship with your partner and your own self development. On the other hand, the less people choose to have children, the faster the population will decrease.
Having children is a great challenge not many people manage to accomplish and to keep your relationship healthy, loving and still as bright. Especially small kids require their parents’ undivided attention, and since nobody has ever taught anyone how to properly raise your children, you will have to find that out all on your own with your partner. So no wonder more people choose not to have children in order to avoid problems it might cause. Also such tendencies may show that people become more aware of their actions and what they can cause.
The disadvantage is that this choice becoming the majority (90% and over), can start the depopulation. This will further lead to economic struggles which will only pursue the popularity of choosing not to have children as it will become economically unprofitable to have children. This will create a loop and might even develop into a world crisis people will not be able to recover from.
Generally speaking, any situation that can have a significant negative impact on the world economy will lead to chaos. If the numbers of such tendency, despite their increase, will not cross the point of 50-60%, such a choice as not to have children will not bring any sustainable effect on the society as a whole.