Demography is not quite as simple as it looks. The key figure, everyone know, is that a population replaces itself if the birth rate is 2.1. However, what does that "2.1" measure?

This is the "total fertility rate" or TFR. This is "an estimate of periodic fertility and is defined as the sum of number of children born by women in a defined age range (16-49) extrapolated to the lifetime fertility of the total number of women in that age group". (I’m quoting Edward Hugh here, a Barcelona based economist of British extraction).

The important thing to remember is that TFR is an estimate, because most of the women in the age group have still not finished their child-bearing life. But, they might surprise us, although it is unlikely that an army of 35-year-olds will have 10 kids, sending the fertility rate soaring. But a 38-year-old might have 2 children. And a 40-year-old might have one.

So the accurate figure is the cohort fertility rate (CFR), "the average number of children which women actually give birth to during their lifetime fertility cycle, [which] is only known when the women in the cohort end their fertile life". In other words, we know the number of children that women born between 1960 and 1965 will bear, because their reproductive life has drawn to a close.

This helps to explain the mysterious little bumps in the birth rates in France, the UK and the Nordic countries. More and more women are delaying child-bearing. When they do have babies, it appears to create a mini-baby boom. What’s really happening is that after a lifetime of childlessness women are rushing in before their biological clock strikes midnight.

The point is, comments Norwegian demographer Aslak Berg in the very useful Demography Matters blog, "when the average age at birth is increasing, TFR will tend to underestimate completed fertility." He concludes that this means that the picture for Europe is not quite as dismal as it looks. Almost, but not quite:

What’s been happening over the last few years is not that European women have more children but that the fertility pattern or the average age at birth has stabilized which means that the TFR is now becoming much more accurate in predicting completed fertility. This means that the European countries with a comparatively healthy fertility –a European Fertile Crescent from France through the British Isles to the Nordic countries will have their TFR increase to around replacement rate whereas the rest of Western Europe will have slightly better, but still much too low TFR’s. Incidentally, I predict that much of the difference in TFR between the US and the European Fertile Crescent will prove to be caused by the fact that unlike in Europe, the US average age at birth has remained stable for a very long time –this difference will lead to a differently shaped population pyramid but will not affect the overall size of the population in the long run.