What the chart shows: Follow each country’s age-structure from 2000 to 2050. The bars show the percentage of the population that falls into each 5-year age group. For instance, you can see Nigeria‘s continuing baby boom in the longer bars at the base of its pyramid; China and Germany, however, have bulges in the 60-85 age ranges.

How we got here:

Women's education: Better-educated women correlates with lower fertility rates and vice versa, according to Our World in Data.

• In poorer developing countries, women's education and opportunity outside of raising a family lags behind the richer world, resulting in more women bearing children than their developed-world counterparts.
• "Women with choices lead the way to secularization," Eric Kaufmann, a demographer and politics professor at Birkbeck College, told Axios.
• But for most of the rest of the world, fertility rates have been dropping since the 1960s as women have increasingly pursued higher education and found more opportunity in the workforce, says Richard Jackson of the Global Aging Institute. That opportunity continues to expand, further reducing the number of children born to women from the EU, the U.S., China or Japan, as well as delaying childbearing.

Yes, but: Studies have found that in the U.S., women with the highest education — beyond a bachelor's degree — have higher fertility rates than women with only an undergraduate degree. This is probably a result of their greater financial stability as well as more opportunities to combine child-rearing with a career, according to Kaufmann.

Economically, child-rearing costs have skyrocketed in much of the developed world. The 2008 recession hit middle class families hard everywhere, and some traditional financial incentives to having multiple children are no longer relevant.

Modern medicine and sanitation have helped reduce infant mortality in impoverished nations — although many African countries still have some of the highest rates in the world. This has added to the baby boom seen now in countries such as Nigeria, Kaufmann said.

Religion plays a determining role in fertility rates in both developed and developing nations. Less-religious women typically having fewer kids, and it's common for immigrants and religious women in secular, developed countries to have more children than the norm in that culture, Kaufmann said.

What to watch: Fertility trends could reverse in some nations that continue to grow wealthier and more educated. Richer, developed countries already have comparatively high fertility rates, according to Kaufmann.