Career women are often put in that awkward position of applying for a full time job knowing full well that, as some point in the application process, they are going to have to bring up the subject of flexible working or 4 days a week. There are many other variables to negotiate but, for working mothers, this flexibility is often the most important. SO, what this means is ALL OUR BARGAINING CHIPS are used up asking for flexible working where a man is more likely to negotiate hard on salary. So, right from the start a woman at a senior level is probably being paid (pro-rata) LESS than if she had been a man.
Once this employee has asked for flexibility it is clear she has children and, therefore, responsibility. When she starts she then has to prove her reliability, her ability and only further down the line can she start to speak about the pay increases she should have asked for on being offered the job.
This has surely got to be one of the reasons for the gender pay gap.