Around 40% of pregnancies are out of wedlock. Assuming widespread use say 3/4 of them will be reduced.

This should reduce the TFR by by I don't know 30% and drop the birth rate to around 1.4 to 1.5 in the US

That said, if we want fertility, its going to require massive changes that not to put fine a point on it are politically impossible in any currently configured society.

Democracy, women suffrage, economic liberty (not markets, those stay) birth control pills , trade, immigration , any tolerance of Leftism , hell probably free speech to some degree a lot of those things have to go.

we aren't ready for that and in truth, even if we could pull it off, no guarantee that any of those things will work.

That might be a step too far even for the Chinese. For a sense of perspective, anywhere between 20 and 50 million Chinese starved to death or were killed during the Great Leap Forward and the Cultural Revolution- and that caused permanent scarring within China. Slaughtering 400 million, in a nation of 1.3 billion or so? Even if Beijing's leadership were that galactically stupid, there wouldn't be enough land to put all the bodies.

No problem for China, those 400 million will be euthanized. Unlike the west, China isn't hampered by sentimentalism.