Disappearing children - ethnic group

So, in my on going exploration of the demographics of the area, I've found a break down of the number of children in St Kats and Wapping by ethnic group. The data bands certain ages (for example 0-2) and to produce a straight forward scale, I've divided the number of children in each band by the number of 'years' that the band represents. I've indexed the number of children in each of the 4 largest ethnic groups according to how many children under 12 months old.

We can see two clear groups: ethnic Bangladeshi and other. So although we can see a drop in the relative number of Bangladeshi children as age increases, for the other ethnic groups we can a much steeper drop at age 2-5, such that there are only 20-30 per cent as many non Bangladeshi 11 year olds as there are under 12 month old non-Bangladeshis, where as for the ethnic Bangladeshi population this ratio is about 65 per cent.

Clearly, there will be some interaction between changing fertility and adult population over the preceding 18 years that affects this pattern (more adults, same fertility = more children; same adults, more fertility=more children), but the very steep drop off of 2-5 year olds suggests something very drastic happens either when children get to a certain age, or there has been a massive spike in fertility.

However, it seems on the balance of probabilities that ethnic Bangladeshi families have much lower geographic mobility. Either this is because there is not a desire to leave the community, or there is another factor.