Miller discovered that a "year of delayed fertility leads to a 10% increase in career earnings, a 5% increase in career work experience, and a 3% increase in career average wage rate. The effects are not the same for all women, and women with college degrees, and those in professional and managerial occupations receive the greatest returns. Surprisingly, family leave laws are not shown to alleviate the tradeoff."
Miller is aware that she is treading on somewhat dangerous ground and seems careful to ensure that her research is not somehow twisted to blame mothers themselves for the economic imbalance: "It is no secret that Motherhood may be the remaining obstacle to women’s achievement of economic equality with men (Fuchs, 1988), and deferred motherhood may be a mean of reducing that inequality. At the same time, any financial rewards to motherhood delay are themselves a central component of the work-family conflict."
Food for thought.