Latin America has been consistently reducing its fertility rates during the past decades. It is headed now to a window of opportunity for development that results from "demographic momentum." It is a chance that happens only once, and it requires effective policies to cash in on the benefits it can produce for a population.
Demographic momentum is a period of continued population growth that happens despite reduced fertility rates. It happens because there is an increase in the percentage of people in the population who reach reproductive age, the result of a reduced infant mortality rate that is also usually coupled with the fertility decline. Below you will see six population pyramids, numbered from the least developed country pyramid to the most developed population. If you have never seen a population pyramid before, you have only to know that it represents the number of people in every age group in a given population, the youngest of the population at the bottom of the pyramid and the oldest on the top of the pyramid. 
In effect, this means that the clearest pyramid shape will be seen when a pyramid has a broad base (large percentage of people in the youngest age groups) and a small summit (small number of people in the oldest age groups). You can see this in pyramid 1.
As a country develops and its population matures, its fertility and mortality rates decline. This causes a wavelike motion from the bottom to the top of the pyramid, where the bottom of the pyramid becomes smaller (less people of the youngest age groups), the middle of the pyramid broadens and the top starts to broaden as well. You can see the result in pyramids 3 and 4. Eventually, the decrease in infant mortality, fertility rates and mortality in general will cause the wavelike motion to continue to the top, making the pyramid seem more like a rectangle or even an inverted pyramid, what you can see here in pyramids 5 and 6.
Demographic momentum occurs in populations that look like pyramids 3 and 4. This means that the population may continue to grow despite its low fertility rates because of the large number of people who are now in reproductive age. The two pyramids in here correspond to what most Latin American countries look like right now.
However, this momentum is also coupled to what is known as "demographic bonus." This same population structure means that the working-age population is increased relative to the non-working population. The dependency rate of this population, that is, the number of very young or very old people that are dependent on middle aged people for survival is reduced, giving the population a possibility of strong economic growth and increased savings. The opportunities for accelerated development are obvious.
Nevertheless, the population bonus requires governments to institute appropriate policies to take advantage of it. Investment in education, reduction of gender inequalities, expansion of health services and provision of employment opportunities for the increased workforce are essential. A continued supply of sexual and reproductive health services and family planning alternatives is also indispensable, to avoid an increase in fertility rates that will again increase the dependency rate.
Too often in Latin America we hear conservative groups pointing out that some of our fertility rates are too low for a developing country (such as Chile, with a fertility rate of 2.1, which is the replacement rate for a population). This is a wrong assessment of the situation, since it is based on not recognizing the effects of demographic momentum on continued growth of a population; it also obviates the need for increased investment in social programs to boost development by taking advantage of the demographic bonus. Many Asian countries successfully used this window of opportunity in the late 80's and early 90's.
Latin America still has time left to take advantage of this opportunity. The wide inequalities between countries in Latin America, and especially between wealthy and poor populations in Latin America, have caused the demographic window to take longer to develop than in other regions of the world; it is just starting in some countries and will be fully in place between 2015 and 2030. We are just in time to advocate for investments in development that will benefit from this opportunity—and sustained and well funded sexual and reproductive health services are of utmost importance.
Note: the population pyramids were made using freely available software from the U.S. Census Bureau [7].