From Earth Policy:
Evidence suggests that many women in poor, fast-growing countries would have fewer children if they had the resources and freedom to plan the number and timing of their births. An estimated 215 million women in the developing world do not have access to the family planning resources they need. Worldwide, approximately 40 percent of pregnancies are unintended. A study from the Futures Group and calculations by population expert Robert Engelman indicate that if all women were able to become pregnant only when they chose to, global fertility would drop close to or even below replacement level, greatly reducing population growth.
When fertility declines quickly, reducing the number of young dependents relative to working-age adults, countries can experience what is known as the demographic bonus. Governments can spend more per person on public services, families can spend more on each child, and more money is available to invest in economic development. This “bonus” can kick-start a nation’s economy—it contributed, for instance, to the rapid economic development of several Asian countries, including South Korea and Taiwan, in the 1970s and 1980s.