Modelling and projecting the postponement of childbearing in low-fertility countries

Nan Li, United Nations
Patrick Gerland, United Nations Population Division

In most developed countries, total fertility reached below-replacement level and stopped changing notably. Contrary to the almost stationary fertility level, the mean age of childbearing (MAC) is increasing markedly, reflecting a more general ‘postponement transition’ that may correspond to fundamental shifts for society (Lee and Goldstein, 2003, Billari, 2005). The postponement of childbearing, however, includes changes in fertility at all reproductive ages, which are more complicated than the increase of MAC. Using the idea of the Lee-Carter (1992) method that deals with mortality, we show that the changes in the age pattern of fertility can be well described by a single variable, which leads to simple ways of projection. We discuss some application issues using the data from Italy, and describe a condition under which the model is expected to work well for other countries.