Fertility and Poverty I

This is the first in a series of posts exploring the relationships between income levels and birth rates.

Why do poorer people tend to have higher birth rates than middle class people in the US?

Some guesses:
1. Poorer people may face much lower marginal costs for child birth, for two reasons.
1.a. Social transfers: Child birth costs for the poor are covered by medicaid, some subsidies exist for day care, and state universities are very cheap, especially for those in need.
1.b. Middle-class people are socially (if not primarily internally) obligated to provide for their children at a middle-class level, which is much more expensive per child than for poor people.
2. Poorer people may have a fundamentally different set of preferences and values than middle class. They may be less on-board with the societal materialism contest and more interested in family.
3. Poorer people tend to be the ones who are least equipped to act strategically in the modern world, so they may be more prone to having "accidental" births, or less thorough in thinking through the consequences of having more children.
4. Poorer people tend to work less hours per week (is this true?) and therefore have time to contemplate having children.