> We know that better living conditions (health, income, education etc) lead to lower fertility

How do you come to this conclusion. We're seeing that our oh so clever selves have used chemicals/plastics in these nice living conditions to the point they have negative consequences on our health. Having a nice place to live with a job with a nice salary while lending to better health does not lower one's fertility. Maybe these people with the nice jobs and nice places to live are choosing not to have kids which become the reason they can't have nice things. I think you've jumped to an incorrect conclusion

This trend has been going on for much longer than the current worries about microplastics and whatnot. Lower fertility doesn’t necessarily mean lower physical fecundity. It can also just mean that generations of kids have been raised to believe having kids early ruins your life, and should only be done much later after you graduate university and your career is well established (by which time you’re in your latter 20s and your fertility is naturally lower.)