The National Bureau of Statistics said the total number of people in China dropped by 2.08 million, or 0.15%, to 1.409 billion in 2023.
That was well above the population decline of 850,000 in 2022, which had been the first since 1961 during the Great Famine of the Mao Zedong era.
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.