The thing that seems to boost fertility most is subsidized child care. By cutting the cost of combining work and motherhood, this encourages both. Subsidized nurseries were pioneered in France, a country that has worried about national vigor ever since it was thrashed in the Franco-Prussian war of 1870. It has been rewarded with one of the highest fertility rates in Europe. Cheap nurseries have also helped boost Quebec’s birth rate from one of the lowest of all Canadian provinces to one of the highest.
The cheapest and best way for rich countries to rejuvenate their populations would be to allow more immigration. Migrants need not be coaxed into being, and can start work straight away. However, an influx large enough to save, say, South Korea from demographic collapse would be politically unfeasible. Hence many rich countries (and some middle-income ones like Iran and Turkey) are trying to persuade their citizens to procreate more enthusiastically.
The US had 3.93 million births in 2014 The USA had about 1 million legal immigrants.
The United States remains a popular destination attracting about 20 percent of the world's international migrants, even as it represents less than 5 percent of the global population. The total number of people born in another country in the USA is 41.5 million. The number of people legally immigrating into the USA is about 1 million each year.
The overall world population growth is slowing and could peak in 2050 (unless African birth rates do not slow as much as the midrange UN scenario and then world population could keep increasing through 2100.