They are only shocks if the fertility rate fluctuations have a high amplitude and period longer than ~20 years. For example, if the rate varies between 1.8 and

They are only shocks if the fertility rate fluctuations have a high amplitude and period longer than ~20 years. For example, if the rate varies between 1.8 and 2.4, on a 20 year time-scale, then they won't be "shocks", but mere gentle waves.