The reason why women stop having children in wealthy nations is that the pension system is based around forced collectivism of parental investment.

When you have a working age child in Germany the child's pension payments are added to a common pool that anyone, including the childless can draw from. You might argue that people have contributed their own payments to their pension, but this only works if most people have children of their own. The way the pension system is set up rewards free riders and discourages parental investment from both father and mother including step parents. One of the biggest reasons there is a single mother epidemic is that there is only a biological incentive to reproduce and no economic incentive to raise children. This means as a man you are better off sowing your oats since that maximizes the biological incentives and minimizes economic costs. Due to the defective pension system there is a strong incentive to avoid child support payments since they do not contribute to the pension of the father even though this should be a logical consequence. Hence you see extreme cases e.g. fathers prefer go to prison to avoid paying rather than work and have everything taken.

Women have to abandon their careers to take care of children which represent a pure economic loss to them, especiallyin the form of power pension contributios, when in reality the future pension contributions of their childre. are what makes their pension possible in the first place. The pension system considers their essential reproductive labor to be worthless despite it being an existential concern for the functioning of the pension system. No wonder you have women complain about gender pay gaps and the double burden of work and child care and female pensioners living in poverty.

Then there is the whole step fathers thing. Being a step father sucks, but men have a choice here, so they obviously decide to avoid single mothers in face of the irrationality of the pension system. If step fathers could gain a higher pension for raising step sons and step daughters, then the economic incentives would reduce single parenthood through more step fathers or through more parental involvement of the biological father who will obviously lose out on his pension due to absenteeism.

The ideal pension system allows parents to receive a portion of their children's pension contributions as their own contribution. This means there has to be a free for all pool and a parents only pool. If you are childless you will get a pension, albeit a lower one. If you have many children, your pension will be higher if they are economically successful.