People aren’t comparing today to the ancient past. They’re by and large, comparing life to when they grew up.
So on HN, that’s the 70s through the 90s.
Since then maternal mortality in the USA has only increased. The reasons vary, but this still supports your comment as less people ought to be willing to risk having more children.
I would add, that this stat also pairs with the prevalence of hard pregnancies in the USA, so people may become more protective of their children since they suffered more to have them.
Other countries don’t have this issue, and also have more free range child allowances (e.g. Japan with low mortality, low number of children, but highly independent children supported and watched by adults in general and not their parents).
She may theoretically birth another child but there's a substantial risk.
There are "physical aspects", just because it's all theoretical and principles to you doesn't make them go away.