At this stage in development, it's less about income level than it is the enfolding of the entire domestic sphere into the commodity system and the dissolution of social systems beyond the nuclear family. Children in advanced capitalist countries are an enormous financial burden because every single part of their rearing has been capitalized and commodified. The reproductive sphere has been almost eliminated, and the declining rate of profit means the costs of all that capitalized reproduction are climbing far higher than wages. It's a loss of female agency of a different kind than women in underdeveloped countries have. Instead of being compelled by patriarchy to have children, they are compelled by neoliberalism not to.