Demographic trends show a billion people have lifted themselves out of poverty in the past 20 years, the report found. But swiftly expanding populations, particularly in parts of Africa, could halt the decline in the number of extremely poor people in the world, and it may even start to rise.

“Population growth in Africa is a challenge,” Gates told reporters in a telephone briefing about the report’s findings.

It found that poverty in Africa is increasingly concentrated in a few countries, which also have among the fastest-growing populations in the world. By 2050, it projected, more than 40 percent of world’s extremely poor people will live in just two countries: Democratic Republic of the Congo and Nigeria.

Asked about the best ways of tackling the growing population and poverty challenge, Gates said improving access to birth control was key, and this should be combined with investment in young people’s health and education.

“The biggest things are the modern tools of contraception,” Gates said. “If you have those things available then people have more control over being able to space their children.”

The report, entitled Goalkeepers, tracks 18 data points on United Nations development goals, including child and maternal deaths, stunting, access to contraceptives, HIV, malaria, extreme poverty, financial inclusion, and sanitation.

In its family planning section, the report called on policymakers to empower women to exercise the right to choose the number of children they have, when they have them, and with whom.

According to U.N. data, Africa is expected to account for more than half of the world’s population growth between 2015 and 2050. Its population is projected to double by 2050, and could double again by 2100.

Yet if every woman in sub-Saharan Africa were able to have the number of children she wanted, the projected population increase could be up to 30 percent smaller, said the Bill and Melinda Gates Foundation’s annual Goalkeepers report.

This would also enable more girls and women to stay in school longer, have children later, earn more as adults, and invest more in their children, it added. Smaller families tend to be healthier and more productive.

“To continue improving the human condition, our task now is to help create opportunities in Africa’s fastest-growing, poorest countries,” the Microsoft founder and his wife wrote in the report. “This means investing in young people.”