Before WWI it seems that Britain, France, and Austria-Hungary had low growth. Japan, Germany, and Italy had modest. America and Russia had high growth.
In OTL, even after WWI Germany/Italy/Japan remained above Britain/France in TFR. America remained high as well. Russia imploded amidst murders, wars, famines, and a rapid collapse in fertility.
How might things change without a major war?
Will Germany outstrip Britain and France’s combined population by 1939?
Can Russia get a majority in Central Asia or any other areas of interest?
Will America keep its borders open longer without the war and rise of communism?
Will the Ottoman Empire undergo rapid population growth?
Other?