ITS cities are growing twice as fast as its villages; the slums twice as fast as the cities. Bangladesh is the world's most densely populated large country. The capital, Dhaka, is the fastest-growing city in the world. According to standard population projections, there will be another 90m mouths to feed before the population stabilises, perhaps as early as by the middle of the century. By then, Bangladesh, the size of the American state of Iowa, is likely to be home to about 220m people—or, about the total population of the United States, in 1980.

On March 15th, Bangladesh kicked off its fifth population census since independence. The most basic of its several goals will be to answer a simple question: How many Bangladeshis are there today?

Ahead of the count, all that demography buffs, government statisticians and politicians can agree on is that Bangladesh's population is bigger than Russia's (142m) and somewhat smaller than Pakistan's (185m).

According to the adjusted 2001 census figures, Bangladesh's population stood at 129.3m (an initial count put it at 124.4m; an adjustment for the standard rate of undercounting then boosted the figure). Those familiar with the census mechanics tell of a muddle, marked by “multiple technical problems” starting with some official's decision to procure inferior paper, which fouled up the optical-scanning process…which in turn undermined the quality of the data set. This time, donors are handling the pens and paper—the EU is chipping in over €10m ($14m), or more than a third of the total cost of the census.

The Bangladesh Bureau of Statistics' population clock claims that, at midnight today, that number had risen to 150,220,172. But many think the clock is running too slow. Bangladesh's statisticians have almost certainly underestimated the natural population growth since the last census, according to the International Centre for Diarrhoeal Disease Research. Researchers at the Dhaka-based international research institution—it has been monitoring the country's population for 40 years and has the longest-running and most comprehensive demographic data in the developing world—put Bangladesh's current population at 162m.

The United Nations Population Fund (UNFPA) agrees, almost to the letter: it put the population at 164.4m in 2010. It is the UN body's estimate that has enraged Bangladesh's politicians, some of whom care about these things. A.M.A. Muhith, the finance minister, has called the UN estimate—which suggests that the government may have 14m citizens it would appear to prefer not to have—“condemnable” and “unauthorised meddling”.

The difference reflects UNFPA's pessimistic assumptions about the speed of fertility decline. Helped along by one of the world's most expensive fertility-reduction programmes, Bangladesh has seen a dramatic fall in its total fertility rate. In the late 1970s, women had seven children on average; by the early 1990s just over three. The fertility decline settled at a plateau in 1993-2002, but has resumed sliding since. It has not, however, made up for that lost time. In 2010, the year Bangladesh' s National Population Policy aimed to achieve the replacement level fertility of 2.2, it still hovered at 2.5.

To account for the decade-long stagnation in the rate of fertility decline, in 2004 the UNFPA raised its 2050 population estimate by 25m to 243m. Bangladesh's most recent Demographic and Health Survey calls this “unduly pessimistic” (the government puts its estimate of the 2050 population at 218m). Indeed, many demographers shared the government's criticism and believe that the UN's projection (243m in 2050) was simply too high because it chose to project from 1991 census data instead of the latest data, from 2001. In its latest report, “The State of the Population (2010)”, UNFPA practically reversed its own revision. It now estimates that the population will hit only 223m by 2050, which puts it a mere 5m souls above the government's own best guess. Mr Muhith is reported to have complained about the “honourable clerks” who seem to spend too much of their time sitting behind desks in New York.

Arithmetically speaking, it is a battle over the size of a denominator—many indicators of economic development are expressed as a proportion of the total population. Politically, a small population is a nice thing to have. This is because the smaller it is, the more impressive Bangladesh's progress on the UN's Millennium Development Goals (MDGs) will look (and the warm feeling would be mutual). This refers to progress made towards the goal of halving the proportions of poor and hungry people. So the size of the population matters, either directly or indirectly, for it serves as a denominator in the vast majority of indicators by which progress on the goals in the MDG framework are measured.

But trying to look good on economic development may not be the only reason for the tussle over numbers. If the census throws up a low number, everyone who doubts Bangladesh's bean-counting bureaucrats and their political masters will be asking where those millions of “missing” Bangladeshis have gone.

That question should be taken facetiously: the difference in question reflects different assumptions about fertility and mortality, not actual migration.But given Bangladesh's current relations with its giant neighbour—and its odd geography—this inevitably leads to someone's asking: how many of the “missing” Bangladeshis may have successfully made their way across the bloody border with India? India's 2001 census shows that 3.1m people or nearly two-thirds of all migrants from India's international neighbours came from Bangladesh. That number, like the one from India's 2011 census, is unlikely to be accurate. By many estimates, more than 15m illegal migrants have entered India from Bangladesh since 1971. India's Hindu-nationalist opposition party has been trotting out the round figure of 20m for years.

Bangladesh's demographics are at the heart of what India sees as an interlinked triangle of security concerns. Bangladesh is a haven for various insurgent groups fighting Indian rule in the north-east; what to do if it ever turns away from its secular and tolerant traditions towards Islamic extremism, and, in the process, starts to export terrorist violence; and finally the possibility that large numbers of illegal Bangladeshi migrants are changing the ethnic and religious character of India's border areas.

The quest for a plausible denominator is likely to continue. As for Bangladesh's population clock, no one will have to worry about a reset just yet. The final census results will not be known before December 2013.

(Picture credit: Wikimedia Commons)