OP do you have the same desire to have a third child as you did to have your first and second?
For me this was quite telling. I definitely had a long time (a couple of years) around my 40th year when the idea of a third child was very much up for debate. My partner was not pushing for it but was happy to try if it's what I wanted. And I often thought it was but when it came to it i never could take the plunge.
I had too much holding me back - a growing age gap between the eldest and any third (it would have been almost 7 years plus at the point we thought about trying for a third), the worries about the unpredictability of conceiving/ carrying / birthing another child, worrying about moving to a coping situation rather than a happy and settled one if we add a third child, worry about multiples (a higher risk with fertility treatments), etc.
I didn't feel that way with my previous pregnancies and desires for another child. I have been pregnany more than twice and have needed fertility treatments to get pregnant. I have two children which I way more than I event dreamt I would at one time.
In my case i think it was a combo of my youngest growing up and my own age and hormone shift that led to the issue coming up at all.
If I had my time again, I may have pushed on for a third much sooner but at the time the thought of having 3 under 5 was not good as mine were both v poor sleepers. Had I had my first when I was late 20s, a larger age gap between 3 or even 4 wouldn't have felt so bad. And maybe if I had conceived and birthed easily each time with no losses I may have had that third with no hesitation.
But ultimately it was self awareness that I would be a less good mum to three children and my DP would be a less good Dad because of the additional emotional capacity etc needed.
The image of me as a Grandma with four or five adult children and who knows how many grandchildren does appeal (environmental concerns aside) but in the short and medium term it is not right for our family to grow to that size.
I do think environmental concerns have to be a consideration when having a family. Where to draw the line on family size feels harder to say.
I think we have to have other ways of providing for an ageing population beyond just making more people. A better taxation system would be a start.