Some people have no imagination. We are speaking of totalitarian regimes with the might of the Soviet army behind them. OTL they did incredibly idiotic things and got away with it.
Meaning if the party decides the country needs more people they will try to do it. Like dont know, make it a law that every women must have at least 4 children by the age of 30. The only exception is if they are infertile. If they dont than that has consequences. A milder version is to forbid abortation - this actually was law in Hungary for a few years under Rákosi. Or other idiocies they may come up with.
We're speaking of totalitarian
Marxist-Leninist regimes (I suppose Titoism and Hoxhaism can be included under that banner), which have a particular ideology regarding women's rights and the role of women in society which doesn't make sense to alter too much. Eastern European regimes also had increased levels of female participation in the workforce and female education. Now, these states obviously weren't paragons of feminism, but they had inherent reasons (along with their economic strengths versus undeveloped African/Asian countries) why such high fertility rates are unreasonable.
The other factor is corruption. You can pass laws like Romania did demanding women reproduce, but can you actually enforce them? History shows us that, no, you can't. There's too many people you can bribe to make the problem go away, and the government is none the wiser or you have corrupt officials complaining about other corrupt officials. To use your example of "every woman must have 4 children by age 30", I'd imagine there'd be lots of bribery that would create children who existed only on paper, or perhaps abduction/trading of children from parents with more children (or less) or stealing children from orphanages--it's something which would be circumvented.
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This graphs shows a comparison between Poland and other developed countries. These developed countries were chosen because they had higher TFRs than Poland thru out the latter part of the 20 century.
Green: Shows countries in the Americas.
Yellow: Shows countries in Europe.
Red: Shows countries in Asia.
Argentina's post-WWII fertility rate shows the decline of the Argentine economy due to national instability which prevented it from following a trend akin to more developed countries. Israel has a large amount of highly religious Jews who have correspondingly huge families. The situation isn't comparable, especially with Israel.
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Albanian TFR spiked during WW2, peaked in 1958 at 6,54 children per woman and did not start to decline until 1959. Perhaps this decline could be avoided or lessened?
Albania was a highly impoverished country, with the lowest GDP in Europe. The country was extremely unstable for most of the 20th century until Hoxha's iron-fisted rule was established. It's logical it would have a high fertility rate for the same reasons African countries do.
The fertility rate and population increased because the Albanian government modernised the medical system to whatever degree they could as well as were able to eradicate many diseases like malaria. At the same time, Hoxha's regime also restricted abortions. However, in other ways, Hoxha's regime was very much in favour of women's rights, as communist regimes have tended to be, by encouraging female education and advocating for women to join the workforce. Quote Hoxha "The entire party and country should hurl into the fire and break the neck of anyone who dared trample underfoot the sacred edict of the party on the defense of women's rights."
Combine nominal/on paper support of women's rights common in Eastern Bloc regimes with the fact the government can't be everywhere at once, and you have a system which will prevent artificially high fertility rates from existing.
