Figure 1.

Timing of influenza pandemic and birth rate reduction in Denmark, Norway, Sweden, and the United States. A, Time series of monthly natality rates in Denmark during several decades surrounding the 1918 pandemic. It can be seen that an unusual and unseasonal birth rate depression occurred in early 1919, followed by an equally unusual increase (compensation) from late 1919 through early 1920. The natality patterns are typical for natality data from each of the populations studied. B, Time series of residual natality rates after subtracting out the expected (modeled) baseline, contrasted with the peak influenza (flu) periods, for Denmark (DK), Norway (NO), Sweden (SE), and the United States (US). For each study population, the severe autumn 1918 peak was followed by a temporary period of compensation 6–9 months later. This decline is attributed to first trimester miscarriages among a subset of all pregnant women who had pandemic influenza. The shaded areas represent the 95% confidence intervals P&I deaths indicate respiratory mortality due to pneumonia and influenza.