The effect of changes in timing of childbearing on measuring fertility in England and Wales.

Changes in the ages at which women give birth to their children mean that fertility measured at a particular point in time (period) may not be a good representation of the ultimate fertility of those women. The common measure of period fertility is the total fertility rate, which in 2001 has fallen to the lowest level since records began in England and Wales. This article presents various methods that have been proposed to adjust period fertility data to take account of changes in the timing of childbearing, applied to England and Wales data. The article concludes that while these adjustment methods provide useful insights, for example, that the total fertility rate has underestimated period quantum fertility since the 1970s, the measures produced are difficult to interpret. This is in part because the concept they are trying to measure, period quantum is itself imprecise. The adjustments do not necessarily provide a reliable indicator of underlying cohort fertility.