> I'd speculate this is because if you are either rich or poor you have stability in that your life circumstances aren't going to change that radically,
In the U.S. this may be due not to 'stability', but Medicaid.
To be clear, Medicaid benefits are a hot mess of bureaucracy. To paraphrase Voltaire: 'The point of Medicaid is to ensure that no two people are covered under the same set of rules'.
That said, Medicaid covers ~22% of the U.S. population. The Medicaid yearly income cut-off for a 2-person household is ~$57k/year. Most of the costs of a pregnancy are covered under Medicaid [0].
What are such costs? Well, again, it's very complicated, no birth is 'standard'. And googling this is going to lead to a wide range of answers. But in my local area for a first birth, one baby, no c-section, no epidural, no emergency measures, you're looking at ~$20k out of pocket. And that's with a 'good' HCP. C-sections go to ~$80k, and those are ~20% of first births in my area. Again, the costs will vary wildly depending on many factors.
Then you add in check-ups, vaccinations[1], standard illnesses, ear infections, etc. Medicaid will mostly cover these things, while anyone making more than the cut-off must pay them out of pocket with their HCP discounts, if applicable (again, caution, it's super complex bureaucracy).
The interesting thing is to look at other countries with mostly functional universal healthcare systems. What has happened to their birthrates based on income when you look at how much a mother can be expected to pay for a birth? From what I can recall, there seems to be no real difference as compared to the US, correct?
[0] https://www.growingfamilybenefits.com/medicaid-coverage-duri...
[1] They may be free, but it depends on your HCP. Sometimes the nurse to inject will bill pretty high. I've gotten ~$300 bills for vaccinations before.