Except it's not actually about economic pressure, not really. Calories are cheaper than ever. One could theoretically obtain an iron-age tier way of life with 1/100th of the effort iron-age people expended for the same thing.
It's actually about regulations and status games -- and how they work together. In lieu of actual scarcity, we have psyched ourselves out into a simulacrum of scarcity by endlessly revising the list of things that are considered "necessary for life," and have moreover shaped the moral authority of the state to enforce these ever-changing (and very costly) standards.
Because though calories are cheap and basic building supplies are plentiful, it'd be social suicide to live an "iron age lifestyle" with 8 kids. Your living structure would be illegal, your living standards would justify a CPS investigation and possible abduction of your children, and every single person around you would condemn you as a lunatic. Eventually, even if you could avoid these fates -- your kids may very well disavow you and opt not to repeat your way of living.
These days, to have children you are "supposed" to be spending a lot of money on a suite of recently-invented technologies, behaving exactly as your neighbors behave, and presumably working constantly to finance this incredibly expensive social status game.
The sum total of building codes, immense social pressure to conform, ever-changing 'rules' about what is necessary for life, infinite consumerism, wide-latitude child welfare policies, and a few other distinctly modern factors have created SIMULATED scarcity that is far more intense than any iron-age peasant ever felt or knew.
Only countercultural minority groups will make it through this bottleneck. I'd even dare to say that if you even slightly care about "keeping up with the Joneses" your bloodline is probably not gonna make it through the next century or so.
Birth rates are declining for 1 simple reason: scarcity. There are too many people and they all need water, food and shelter. And so, we now spend 90% of our time working for the economy (other people) and have 10% of time left to establish a family.
In the year 500 AD, you didn't need to save up for a mortgage. You simply got a plot of land as a serf and started having kids effectively right away. Nowadays, you need 20 years of education, save up for another 10, and maybe you'll have a 1-bedroom studio but no room for a kid.
The reason behind all this is economic pressure. You're spending your vigor on working for the economy (other people) instead of for yourself (a family).