The largest factor encouraging people not to have children (preventatively or abortively) is money. Taking time off work is akin to dropping down below poverty level and childcare sometimes makes working a net negative financially. Housing, healthcare, schooling... it all traces back to money. Legally you're supposed to have your position at work protected but in most states you have to burn your vacation time and in practice you're penalized for being out (enforcement requires you sue your employer which is not easy and makes finding future employment difficult as the lawsuit shows up on "background" checks marking you as a "problem".)
Free universal childcare, free universal child healthcare, legally-mandated accommodations with government enforcement for childbirth (including both work and schooling) would massively change the equation for many people who currently choose not to have kids. This must be accompanied by programs to train people to operate daycare centers, graduate more pediatricians, and so on.
I also need to clarify: I don't mean "means-tested" programs like Medicaid where taking a job disqualifies you from the program or a childcare program where means-testing excludes most people. Or "welfare" programs that require you file 12 TPS forms every 2 months or they kick you out. I mean low-hassle automatic "proof of citizenship or legal residency = automatically approved, no questions asked".
And by "legal accommodations" I mean any student wanting to give birth gets an automatic 5 year sabbatical with no penalties from high school, university, grants, post-doc programs, residency programs, etc. The government must handle the reporting, auditing, and enforcement so it doesn't become a "legally yeah but in practice no" like we have today.
If those solutions seem complicated the simpler route is a large increase in wages plus import controls/taxes or other measures to penalize offshoring to avoid pro-family wages. Most people don't make enough that even a 100% refundable tax credit would make up the gap but it might help.
In any case these are all large interventions with costs. People are often reluctant to pay those costs (in taxes or higher prices for goods).
I'll admit using government to restrict personal freedom (banning contraceptives/abortion, banning proper sex education, etc) is definitely cheaper, though I find that morally objectionable. In practice it isn't all that effective either.
edit: having a developed economy is technically the primary driver of lower birth rates but only up to a point (5-8 down to 2-3). When people feel financially secure and secure in their career they tend to average out to replacement rate (give or take). The calculus for a subsistence farmer is quite different from anyone likely reading HN.