I suspect high income >200k/yr households are also highly correlated to high CoL locations. If you live in an SF 1 bedroom that costs 1.5 million dollars the marginal cost of an additional child is likely to be over a million dollars in the first 10 years of life. A family with exactly 200k/yr is at the entry point for financially achieving what would have traditionally been viewed as a middle class financial situation in the US e.g. owning a home, funding a retirement, taking regular vacations, paying for children's college etc. Even 1 child in a high CoL location can jeopardize these goals.

A relevant number would be the difference between income and the marginal cost per child.