Ok, read carefully.
Before the Tzunami, earthquakes and nuclear near meltdown, statistics in Japan showed that the death rate (and other factors) are larger than the birth rate. Which means that their population would naturally decline throughout the years. the government even issud incentives for young couples to get married and have children, but the tred didn't change. This resulted from the unique culture japan has and the way of life there.
On the other hand, there are countries that have a an average birth rate much higher than the rest, for example in Africa you have the democratic republic congo, Guinea-Bissau,Liberia and Nigeria, followed by a 'middle eastern' country, Afghanistan. These are the top 5 countries with the highest birth rate in them, compiled by the UN. One can recognize the pattern behind them and see the similarities between them all.
One cannot try and suppress the population growth by imposing a law, because population growth depends on the life quality, culture and society of different countries, and that changes drastically between different countries.
this is unnecessary cruel to prevent families from having more than 2 children while you do nothing to prevent the tribal people in the democratic republic of congo to only have 2.
You can do nothing to prevent the impending doom.
Brace yourself for the inevitable doom .