Nothing (that aligns with classically liberal values). Offering child rearing incentives like free childcare, education, outright money, etc does not seem to work, such as in the Nordic countries. People who grow and experience wealth simply don't want to have kids.

If you truly want to raise the birth rate, at any cost, you would reduce education levels, make people poorer, raise religiosity, and ban contraception and abortive services for not just women but everyone. But your country's economy would suffer over and above the slowed birth rate you'd otherwise have.

In the future though, with artificial wombs, I'd imagine we'd go through a Brave New World type scenario where the government grows children themselves (and maybe has the bright idea to stratify the children's gender, race and intellectual capacity as well, but the latter is unlikely if we have AI and robots that do the jobs instead of needing perpetually content Street cleaners).