It's simple biology that as you approach menopause, there's an inbuilt evolutionary response that makes you want to have one last roll of the dice.
I felt similarly in my early/ mid 40s. After I met DP at 42, we talked about it and decided that we probably wouldn't be ready to take that step for another 3-4 years, and that 46 was probably too late. Plus when it came down to it, neither of us desperately wanted another child. Would like one, yes. But desperate yearning, no.
I had a couple of friends who tried for a much wanted first child in their 40s and sadly were unsuccessful, and suffered several early losses along the way which was very traumatic, and seeing how painful it was for them was another factor against.
That said, I always planned to have 1 more child. I never meant for my youngest child (now 20!) to be my last. Had I not met DP when I did (I was at the point of giving up on dating and men entirely), it's possible I might have tried to get pregnant by donor, it was something I had considered (and another couple I knew had done successfully). But would I have regretted it? Almost certainly yes if the outcome had been anything less than optimum. I know that I would not have been able to cope with a child with any form of additional needs. It could have turned out wonderfully; equally I may have been filled with regret.
Part of me will always wish I'd had that last child in my 30s, that I'd met DP sooner. But things turn out as they turn out and for all that I would have liked one more child overall I think I made the right decision for me, for my relationship and of course also for the children I already have.
And of course none of us knows how fertile we are in our 40s. My grandmother and her mother both had children in their 40s, but who knows if my eggs had not long since withered?.