Both Russia and Ukraine have dire demographic outlooks and their pyramids show waves due to previous calamities, but indeed the political rhetoric in Russia is 'make more babies', also a reason to steal kids from Ukraine. Regarding the long term, there is indeed a correlation poverty - fertility but that doesn't imply causality - other related factors such as urban / rural and education can be more relevant (kids need a lot of space and time). However iirc the earliest models such as limits to growth did show a long-term oscillation due to the feedback you suggest.