This year, London is expected to reach an important milestone: It will finally surpass the population it had back in 1939, just before World War II, according to Britain's Office for National Statistics. When the city's population reaches 8.615 million, the British capital will be the largest it's ever been.

London's post-war slump may come as a surprise. The city never became a ghost town, and even as Britain began to lose its clout on the world stage, the city remained an important political, economic  and cultural hub.

But, as this chart made using historical data from ONS shows, the post-war exodus was very real. As Barney Stringer of City Metric has noted, the population dropped by more than 2 million people, more than twice what Detroit lost over the same period.

The war was, unsurprisingly, a key factor here. Many people, both soldiers and civilians, were killed during the war, and hundreds of thousands more were evacuated to the countryside. Vast areas of London, in particular the East End, were nearly destroyed during the blitz. The war quite simply changed the fabric of Britain's capital.

After the war, London suffered from a trend first witnessed in America, what was referred to as "counter-urbanization" by human geographer Brian Berry in the 1970s. People were choosing more and more  to not live in cities. In London's case, many chose to live in the city's sprawling suburbs and the "new towns" first created for those displaced by the war.

However, by the mid-1980s, that began to change. London's population began to rise, slowly at first, and then much quicker. Predictions from the ONS say it will reach 10 million within 15 years and keep rising after. London's reputation as an international city continues to grow: It may no longer be the center of the empire, but it's now the largest city in the European Union.

But London's growth will bring with it numerous difficulties. The city is already notoriously overcrowded and expensive, and transportation has become a major issue in a city of its size. A recent report commissioned by the mayor of London found that the city needed to be proactive to keep its status as one of the world's top business cities, in particular reaching out to potential migrants.

That report belied one of the realities of London's growth: It has not been white, British people driving it. In fact, between 2000 and 2013, 620,000 white British people moved out of the city. As the BBC notes, that's larger than the population of Glasgow.

Statisticians from the Greater London Authority are expected to announce the moment when London becomes the largest its ever been in the next few days. According to the Evening Standard, the record breaking baby is expected to be born in one of the outer boroughs, where growth is highest.