Most of Europe is below replacement level and is still going down. Since this has never before happened in history we don't know what the floor might be or if there is a floor.
The same trend is affecting Americans of European ancestry. The only reason we haven't fallen off the cliff yet is birth rates among immigrants - and the State of Utah.
Utah is known as a center of the Mormon faith. Devout Mormons do not believe in birth control.
We could probably write an entire book, about population trends, and reasons behind them.
But the implications for society are profound.
For example, tens of millions of babies have been aborted since Roe vs. Wade. Some think that our illegal immigration problem stems in part from that fact. According to that theory, the fact that people in our native born population were not having as many children as previously, created gaps in our labor markets, which were filled by illegals.
I’ve heard other theories too, along the lines of, people didn’t want large families for various reasons, and reliable birth control allowed for people to have as many, or as few, as they ideally wanted. Surprise 3rd or 4th babies are a thing of the past for significant numbers of people nowadays.
As civilizations become prosperous they have progressively fewer children. It happened to Rome. In the West with the child labor laws and the determination to let kids be kids children have become a net financial liability whereas historically the more children the more prosperous one’s family. In an agricultural society you raise your labor force at home. In an early industrial society the more kids you have the more incomes from the factory support the family. In a late industrial society the suggestion that one’s children could help support the family draws gasps of horror even from people who consider themselves traditionalists and conservatives. The more expensive children become the fewer there will be. In more traditional less developed societies children are expected to pull their own economic weight by the time they are ten and usually do. In America we just made new laws that forbid farm children from doing “chores.” We raise children to be older children so that we have far fewer emotional adults than biological.
From my own limited observation in the DC Metropolitan area, younger American couples of European descent marry and have families later, into their 30s, and fewer kids, but they are raising having and raising their small families. Today it’s rare in any metropolitan area to see an at-home mother. That’s just tragic IMO.