Bill and Melinda Gates Chris Ratcliffe | Bloomberg | Getty Images

Progress in fighting poverty and disease may soon stall, and could even reverse, after nearly two decades of progress, Bill and Melinda Gates warn. More people are being born in the world's poorest countries, which could halt the decline in the number of extremely poor people and possibly even cause the number to rise, the billionaire philanthropists wrote in the Gates Foundation's second "Goalkeepers Data Report." That would overshadow the fact that more than 1 billion people have lifted themselves out of poverty since 2000. Despite the worrisome outlook, they're optimistic that investing in the young people can help avoid it. Yet this comes as President Donald Trump has vowed to slash foreign aid and nationalism spreads through Europe. The Gates Foundation still has "a lot of concerns" about foreign aid, Bill Gates told reporters on a briefing call. "In the United States, we are very thankful that the Congress has maintained these things are priorities, despite the executive branch recommending very substantial cuts," he said. In its second year, the Bill and Melinda Gates Foundation produced the report with the University of Washington's Institute for Health Metrics and Evaluation to track progress on 18 data points from the United Nations Sustainable Development Goals. They released it Tuesday as the U.N. General Assembly convenes. They released it Tuesday as the United Nations General Assembly convenes.

By 2050, the Gates Foundation estimates more than 40 percent of extremely poor people in the world will live in the Democratic Republic of the Congo and Nigeria. One strategy to address the population boom is to improve family planning and limit unwanted births. "Rapid population growth in the poorest countries, particularly in Africa, puts future progress at risk," according to the report. "If current trends continue, the number of extremely poor people in the world could stop its two-decade decline—and could even rise." Africa's population of 1.1 billion today is expected to double by 2050 and to double again to 4 billion by 2100, according to United Nations' data analyzed in the report. If women in sub-Saharan Africa could control their fertility, that estimate would fall to 2.8 billion, or 30 percent below the current prediction, according to the report. Strategies include helping women gain access to "modern tools of contraception," as Bill Gates called it, such as implants, injections and intrauterine devices (IUDs). And although birth control is controversial in the U.S., the bulk of current funding for it comes from the U.S. and Europe, he said. "So, whenever there's an issue about that money might get cut off, you know, where there's no clear way that something would come in and substitute for that, then all you're doing is making a woman less able to have her desired family size, which both at the local and broad level, you know, we think is very, very unfortunate," he said.