The rate of population growth is the balance between fertility, mortality and migration. The birth rate minus death rate gives the natural rate of population growth. Generally speaking, the natural rate of population growth in the past, until a couple of centuries or so, was very slow, at times negative, because a high birth rate (more or less constant) was matched by almost equally high (often fluctuating) death rate. Coming to the more recent period, the natural rate of population growth in Bangladesh was small during the decades 1901-11, 1911-21, 1921-31 and 1941-51, moderately high in 1931-41, and high in 1951-61. This pattern contrasts somewhat from that observed in the sub-continent of Bangladesh, India and Pakistan as a whole, because the incidences of the 1918-19 influenza death was lower and the 1943 famine deaths much higher in Bangladesh, compared to the rest of the sub-continent.

The rate of the natural growth of the population in Bangladesh in 1951-61 was 2.2 per cent per annum with net emigration the rate of growth was 1.9 per cent per annum.

Since 1943 Bengal famine, there has been a monotonic improvement in the mortality of the population, while fertility remained more or less unchanged at a high level. This resulted in an accelerated natural rate of population growth. Compared to the decline in mortality a model pattern of fertility change is that with a time lag of a number of years (the length of the time lag depending on the relative success of the family planning efforts of the people and the government) fertility start declining, though slowly initially and then quite fast. This simple pattern of fertility change is vitiated somewhat because of the dent in our age distribution as mentioned above. Even if we assume that our population have shown signs of declining fertility in recent years, the population growth rate will not decline during the plan period from its pre-plan period level.

Calculation of the Expected Population Size on January 1, 1973.