That’s not true about the USSR. Birthrate was rapidly decreasing starting in 1910, with a brief post war boom (from like 2.5 to 2.8), then continued decreasing, reaching 2 in 1970, then stabilizing (with a slight dip under 2 for a few years) till 89, at which point it crashes to 1.25 and has been recovering since, now around 1.82.

China’s fertility rate was also decreasing before the one child policy, however the govt viewed it as not fast enough. It peaks in 65 at like 6.4, then rapidly decreases to 2.6 by 1980 when the policy is implemented I presume to force it under 2, which it does by 1992.

The US baby boom was a boost from 2 to 3.5 over 1940 to 60, then a rapid crash back to 2 by 1970, continues falling till the 80s around 1.8, gets back to 2 in the 90s, and then has dipped back down to 1.8 since then.

I’m not quite satisfied with the economic argument for the US boom either, like the material conditions of the 60s were not that different from the 50s, but the availability of birth control was, and the drop from 60 to 65 is massive, and before 1940, increasing industrialization and material conditions had a super clear trendline with decreasing birthrate from 1800 when the birthrate was 7, to 2 by 1940 (with a slightly sharper drop in the 1930s due to the great depression).