Now just for the record, if the Nazis and Japanese had a "Victory" that resulted in them controling all of Europe (minus the UK and Ireland), Asia (Minus the Phillipines and India) and Africa (minus Liberia) you cannot assume that the OTL decline in the birth rate is going to happen since you are going to be dealing with different birth circumstances.
You see lets assume that Hiter does shortly after WW2 and the other Ultra-Facists (Himmler etcc.) are either killed or exiled to a remote part of their domains and Speer and his counterparts are in charge in Axis Europe. Now sure they have "won" the war but they now have a very big problem.
The so called loyal (or “Aryan”) population of the Axis powers is a minority within these 3 continents, everyone else is rather hostile towards the Axis Powers and thus with a few exceptions is more than happy to fight the Axis powers over their occupation of what they consider to be their lands.
Thus the Euro-Fascists are going to face a massive security problem over the next few decades across Europe, Asia and Africa. If they just fight the rebels (who would have a lot of American support) they would waste huge amounts of money and suffer such loss of live that there would be growing discontent locally.
On the other hand in they withdraw from those areas which are too difficult to deal with then they still have a serious security issue since the continent is unsecure and the rebel forces are unlikely to be the forgiving type considering the record of the Euro-Fascists. Not only that but you have no guarantee in retaining control of the access to the natural resources and thus posing a risk to the economy.
In both cases it would mean that the future of the Euro-Fascist elites is put into question. Thus the problem clearly needs to be solved and no you cannot simply use genocide (of course it is not really a good idea to use at all) to simply wipe out populations who have issues with the Euro-Fascists, because it will leave depopulated land which will be a drain to the economy.
Thus the only solution is to populate Poland/USSR/Africa/Middle East with loyal (or “Aryan”) peoples to take place of those who are hostile to the Euro-Fascists (the Slavs and Arabs who lived in those regions beforehand) who are no longer in those areas either because they have been forced out or been killed and the only way you can do that is to keep birth rates high.
Thus unlike Europe in OTL or any developed nation on earth in OTL (were was no reason to have a high birth rate and thus once contraception allowed lovemaking to happen without conceiving children the rate thus dropped), there needs to be a continued high birth rate otherwise the whole system will come crashing down.
For the record if you consider the populations of Germany, France, Spain, Portugal, Italy, Netherlands, Romania, Bulgaria, Norway, Sweden, Turkey, Finland and Denmark (if one is to be flexible about ones "defination" of Aryan, which in itself is a stupid concept*) as well as South African and Rhodesian Whites and Coloreds (Plus additional "Non-Aryans" in OTL considered "Aryans" ITTL) you have 250-275 million to expand upon to settle what needs to be settled. With a reasonable growth rate that should be possible.
Now how this must be done is another matter, considering the importance of having a high birth rate for the future of the ITTL status quo and thus the jobs of the elite they would likely be willing to try anything that would work to have a higher birth rate.
Firstly there is the matter of financial incentives. One of main reasons why people may or may not have children is because of the overall cost. If there is a financial incentive to have children then that barrier to having more is taken away. This financial support should also be extended to fertillity treatments.
Then you have to look at education and propaganda, using the example of Palestine is allows a mindset to be created that having more children is a good idea for the safety of the nation and is a "patriotic" thing to do for your country. I got the idea (for the latter) from a Tom Clancy Novel called Red Rabbit (there is a part were a KGB agent recalls how some of his fellow agents state in great detail about their sexual antics to show "how much they are defending the fatherland")
At the same time you must also use those same methods to encourage people to have sex (hetrosexual, sadly homseoxuals are going to suffer badly ITTL in Europe, shame really) from as you an age as possible (within limits of course), this can include a earlier ITTL introduction of Viagara foe example.
Speaking of which you also need make sure that the "contraceptives" up for sale are modified in such a way that they simply do not work for example.
Lastly you must also make Pregnancy seem attractive though propaganda, this includes going up to the festish level. (there is such a thing in OTL).
Also even if women work in the same numbers as they do in OTL that is not alone a reason for the birth rates to fall, for example in the Ultra-Orthodox Jewish Community despite their religious conservatism many women work because under their sect men are meant to study religious texts and thus reducing their chances of gaining paid work so their women fill the gap, yet the birth rates are still very high. To allow this ITTL you need a decent childcare system in place.
Lastly can I make it clear I am no fan personally of what I have suggested. However considering the circumstances ITTL this is the sort of thing they need to do (if I had my way Hitler would have not nowhere or any of the other Nazis). It makes one think how crazy the Nazis idea were and in my view even if Speer and his counterparts suceed Hitler and actually make some sort of state coming out of the Third Reich and its counterparts work to the present day. History would not look to kindly on him or his sucesssors. In fact considering all the mass murder and other crimes against humanity that will take place, for many people under their rule it would be a living hell and a dark reminder of what humanity can do.
PS: To think of pretty much the only example that comes close you have to look at Gaza for example. In fact you might as well say that the "real" cold war would take place in the bedrooms of Axis Europe and their other territories.
*Hitlers idea of an "Ayan Race" that is and his defination of it as well as his idiotic belief of "Racial Superiority"
