There are currently 7.7 billion people on Earth. In the next 30 years, that number will likely rise to 9.7 billion. By the end of the century, it could be as high as 11 billion. That’s according to the latest World Population Prospects report released this week by the United Nations (UN).

According to the report, the country projected to see the greatest population increase in the coming decades is India. The nation is estimated to overtake China as the world’s most populous country by around 2027. India, along with eight other countries, will be responsible for about half of all population growth between now and 2050. These eight countries are Nigeria, Pakistan, the Democratic Republic of the Congo, Ethiopia, Tanzania, Indonesia, Egypt, and the USA. Sub-Saharan Africa’s population is projected to pretty much double by 2050.

“Many of the fastest growing populations are in the poorest countries, where population growth brings additional challenges,” Liu Zhenmin, UN Under-Secretary-General for Economic and Social Affairs, said in a statement. These challenges include tackling malnutrition and poverty, improving education and health care, and increasing equality. Life expectancy in these poorly developed countries is currently seven years lower than in richer nations, due to factors such as high mother and infant mortality, violence, and diseases like HIV.

In contrast, population numbers in many countries are actually declining. Since 2010, 27 countries or areas have experienced population drops of at least 1 percent. Over the next three decades, a total of 55 countries will see their populations decline by at least 1 percent, with roughly half experiencing drops of at least 10 percent. A key reason behind this trend is falling fertility rates. Back in 1990, there were, on average, 3.2 births per woman worldwide. This has now dropped to 2.5 births per woman and is expected to fall further to 2.2 by 2050.

Migration has also become a major factor in certain countries’ population declines, the UN notes. Bangladesh, Nepal, and the Philippines have seen the greatest outflows of migrants, thanks to the demand for migrant workers from other nations. Syria, Myanmar, and Venezuela have also seen many of their citizens flee due to violence, armed conflict, and insecurity. Meanwhile, in certain countries like Germany, Belarus, and Estonia, immigration will likely offset population losses.

Declines in fertility in various countries may actually be having a positive effect, the UN reports, as it means that the fastest-growing age group is 24-65, i.e. people of working age. This could help to enhance economic growth, allowing developing countries to invest more in education and health care.

In most countries, the proportion of people over the age of 65 is also on the up. According to the new report, by 2050 one in six people will fit into this age group. Today, that number is one in 11. Some parts of the world, such as Northern Africa, Latin America, and Asia, could see the number of older people within their populations double over the next 30 years.

By 2050, a quarter of people in Europe and North America are projected to be over the age of 65. This rise could cause financial strain, as more money is required for health care, pensions, and social protection systems.

“In 2018, for the first time in history, persons aged 65 years or over worldwide outnumbered children under age five,” says the report. “Projections indicate that by 2050 there will be more than twice as many persons above 65 as children under five.”