A mother, when pregnant, has a lower productive value in the workforce, because she is more likely to prioritise her growing family over her career. Meanwhile, a new father is seen as more 'stable and committed to [his] work', because he (unlike his female counterpart?) 'have a family to provide for' and is therefore 'less likely to be flaky'. This results in new mothers receiving less responsibility in the workplace compared to their male counterparts, and consequently receiving lower pay.
If this article is true, then the tragedy here is that the economic disparity between the male and female workforce reflects a discriminatory policy in the valorisation of productivity, i.e. that certain productive processes are valued more highly than others. In other words, we are discriminatory; regardless of our talk of freedom and equality, the economic structures of our world show what our hearts are really made of. This discriminatory valorisation arises from old cultural beliefs that certain productive processes are indeed 'better' than others, whether in the sense of the moral, biological, or even socially 'practical' (in terms of social cohesion).
Further, the valorisation of productive processes is signified by money, which by signifier alone is a form of political and economic agency for the individual, male or female. In other words, if one's contribution to society is perceived as inferior to another's, then one's voice is also. But if human perception is subjective anyway, then so is the valorisation of productive processes. The trouble here is that collective human perception leads to consequences that are very real, very material, to the individual; for example, one's access to food, water and shelter, as well as (indirectly) one's social circle, material wealth and individual autonomy (which one needs for any kind of self-actualisation). When quality of life is in question, when an individual's full potential cannot be realised because of the material hindrances apropos human perception, then something is wrong.
Economic disparity says something about our beliefs and values as a society. I cannot claim that the current approach is absolutely wrong (for there is still the utilitarian argument), but justice is never being done when something becomes established unquestioningly.