I feel the answer might be simple actually. As contraceptives becomes well understood and taught kids are opting to have a better quality of life and not repro

I feel the answer might be simple actually. As contraceptives becomes well understood and taught kids are opting to have a better quality of life and not reproduce. They put it off so they can build a career. When they build a career they get used to a certain quality of life. And so by mid thirties they don't want to have 3 kids. Of course they delay having kids till they are financially stable enough to have kids and not worry about money and hardship. Because who wants to voluntarily go through extreme hardship. And so as cost goes up and pay goes down with available contraceptives people are just not having kids until they have the financial stability which many might not have.