That’s a literal question. What public policy do you endorse that has the ability to significantly reduce the number of people being born?
If you don’t have one you’re just opposed to building enough houses for the people we have.
In the case of the western world, what population growth? The amount of couples in the first-world having children has slowed to a trickle in the last few decades. This seems to have spooked governments in the western world into enacting policies to encourage immigration, ostensibly to prevent a decline in economic growth coinciding with a shrinking population. The only place where the birth rate is actually increasing is Africa.
My personal theory (which people are free to disagree with) is that the decline in birth rate in the first-world is a reaction to overpopulation. Although I acknowledge that extrapolation from this example is risky, we know that some animals lose their desire to breed when population density increases, and in captivity. Is it so strange that humans could be similar?
I'd endorse free contraception.
Options I'd oppose but others might favor include not requiring insurance to cover fertility treatments, prohibiting IVF entirely, mandatory birth control until marriage, sterilization as punishment for crime, and removing the child tax credit. Plus restricting immigration and increasing deportations which have a similar effect on the people/housing ratio.
(But I'm in favor of more people being born, and building lots more housing)
Those things are highly correlated with lowered birth rates, though I suspect the last one is probably not applicable to the US.
None of them are applicable to America [1].
[1] https://www.census.gov/content/dam/Census/library/publicatio...
How much lower do you want it to be?