On October 31, the world population reached 7 billion, a milestone that the UN Population Fund (UNFPA) didn't initially expect until the end of the year.  Looking ahead, the 10 most densely populated megacities face unique problems based on their socio-economic status. 

According to the United Nations, more than 197 million people are living in the world's ten most populated cities. This wouldn't come as much of a surprise, if it wasn't noted that just over 200 years ago, only three percent of the population lived in cities.

Flash-forward to 2030 and more than 60 percent of the population will live in a city, according to a Siemens study.

In 1950, only New York and Tokyo could be classified as megacities-those having more than 10 million inhabitants. Over the last 50 years, the world population has taken off from just 3 billion in 1959 to 7 billion on Oct. 31. In 2010, 21 cities were classified by the United Nations Population Division as megacities, and they are on the rise; by 2025, at least 8 more cities are expected to leap into the classification.

According to a new report by the UNFPA, most of the population growth will happen in developing countries. This change is mirrored in expanding megacities, as cities in China, India, and Bangladesh take over as the most populated cities.

Megacities in developed countries face little to no growth. Tokyo's 2010 population was 36.67 million and by 2025, the UN projects the population to only rise to 37.09. In the same amount of time, Delhi's (metropolitan area consisting of both New Delhi and urban surrounding areas) population will increase by almost 6.5 million people. Growth rates in new megacities often are between three and six times of that in established cities.

The UNFPA found that developing countries often have high fertility rates that hamper development. Cities in transition often can't build infrastructure and afford social services to keep up with the population.

Unlike developing cities, matured cities have already created infrastructure to handle influxes of population from previous generations.

Dhaka in Bangladesh, currently the ninth largest city, has between 300,000 and 400,000 internal migrants flocking to the capital each year. Urban poverty and slums are now the newest problem facing the country.

The World Bank found that while rural areas are currently the predominant areas of poverty, by 2035 poverty will shift heavily to cities. Those living in urban slums face higher child mortality, water-borne diseases, and acute respiratory illnesses. Slums also tend to form near areas others avoid, putting those living there at risk of natural disasters, such as flooding.  

Dhaka was noted as the megacity with the most climate change risks in analysis firm Maplecroft's recent study.  

Population growth in these cities combines with poor government effectiveness, corruption, poverty and other socio-economic factors to increase the risks to residents and business, Maplecroft told Reuters.

Developed urban cities were found to be less vulnerable to climate change; New York and Sydney were classified as having medium risk and London ranked low.

Issues facing mature megacities like Tokyo are aging populations and stagnant growth. The graying population faces welfare challenges as well as issues of economic growth. Developed cities are facing the issue of too few people entering the job market and low fertility rates.  

Population data for the top 10 megacities was compiled using Population Projections of Largest Urban Agglomerations by United Nations Population Division. Discrepancies between population studies are often due to different definitions of city borders.

Click Start to view the world's top 10 megacities along with their current and projected populations.