It only takes a few percent of women to decide they don't want kids for career reasons for the replacement rate to drop below parity.

When you add those who don't want kids or can't have them for other reasons - not straight, asexual, emotional trauma, physically unable, others - getting to parity is even harder.

It's not stress. For a lot of history life was far more challenging, uncertain, and dangerous than life today.

Humans kept reproducing, aggressively enough to compensate for infant mortality, wars, and pandemics.

The big change is that the primary role of women doesn't have to be motherhood, where for most of recent-ish history it was.

I'm not saying a return to that is desirable. But I am pointing out that the causes of low birth rates aren't mysterious.

Women who do choose motherhood are more likely to have kids younger.

But if given a choice, a significant proportion of women will either not choose motherhood at all, or will delay it significantly, which lowers fertility and raises infant mortality.

It doesn't need to be a majority of women. A fairly small percentage is enough to shift the numbers.

I'm not sure. I think there's a lot of people out there who want to be parents, but who put it off in favor of employment because they feel like they need money, and end up having fewer children than they wanted to have. I don't think they're all delaying motherhood because they prefer delayed motherhood.(Or fatherhood for that matter).