Fertility is most strongly correlated with child mortality IIRC. The more likely your kid is to die young, the more likely you are to have lots of kids. Very low child mortality in developed nations, so very low fertility.
Other than the hard work put in to scaring would-be parents. Indeed, the birth control pill put people in control, but it it is only since the "16 and Pregnant" movement, which campaigned on demonizing having children and stressed the importance of not being saddled by a child, that we've seen below replacement levels.
As a result, the youth of today in their prime rearing years look negatively upon having children, and by the time they "snap out of it" they are into their geriatric child birthing years and biologically don't have much time left, putting tight constraint on how many children can be birthed.
> That’s because women choose to study and work on their career in their 20s
Trouble is that those who choose to have children in their 20s have to face the scorn of society. Unless you are strong-willed, you have little choice but to study and work on your career, even if children is what you truly want.
For example — we don’t particularly need to do anything to reduce birth rates of technologically advanced societies societies with human rights. That’s because women choose to study and work on their career in their 20s, while men try hard to save up to afford the sky-high real estate prices. The market does the eugenics for everyone.
If anything, the fear that uneducated populations or those in low-cost-of-living countries are going to “replace us” is a result of that simple fact. Comparing continents, Africa is expected to continue to have a ton of kids per woman compared to, say, Europe or USA.
Religion, of course plays a role. When the condoms are off, the religion doesn’t matter — you’re going to have a lot of babies. The only difference is, how early do you get married? Religions tend to frown on premarital sex.