I've thought about it a bit in the meantime .... In the UK only about 30 children per 100,000 die before age 18 so that would be much less significant than the fact that only about 49% of children are female. Taken together, those facts would imply that you need about 2.0414 children per woman, so 2.05 would be more than enough. So how do we get to 2.1? Perhaps the 2.1 is for other countries with higher infant mortality?
