Table 2.

Temporal Coincidence in the Mean Peak Influenza Month and Birth Depression Peak Month in Each Country Under Study, Using Center of Gravity Statistics

Country, region	Month no. of peak influenza in autumn 1918	Month no. of birth depression center of gravity in 1919	Lag from peak influenza to center of birth depression, monthsa	Mean pregnancy stage at flu peak, months	Cumulative decrease in birth rate in early spring 1919, rate per 1000 (%)b	Cumulative increase in birth rate in late 1919 to early 1920, rate per 1000 (%)c
Denmark, all	11	5.1	6.1	2.9	−1.8 (−7.3)	2.0 (+8.4)
Copenhagen	10	4.5	6.5	2.5	−1.9 (−9.1)	0.8 (+3.8)
Rural	11–12	6.1	6.6	2.4	−1.3 (−5.3)	1.7 (+6.8)
Norway, all	10	4.8	6.8	2.2	−2.4 (−9.8)	2.1 (+8.9)
Sweden, all	10	5.0	6.5	2.5	−4.0 (−13.5)	5.8 (+19.9)
United States, 10 states d	10	4.6	6.6	2.4	−1.8 (−7.6)	0.3 (+1.3)

^aThe lag period (in months) was calculated as the difference between the month of peak influenza and that of the center of gravity of the birth depression.

^bThe dip percentage was calculated as the sum of the negative excess birth rates during the dip period divided by the expected annual birth rate.

^cThe percentage of compensation was calculated as the sum of the positive excess birth rates during the compensatory period divided by the expected annual birth rate.

^dThe 10 states included were Connecticut; Pennsylvania; Maine; Massachusetts; Michigan; Minnesota; New Hampshire; New York; Vermont; and Washington, D.C.