If you look at the senior facuilty member of Harvard Math department, there is only two female professors (out of 26 listed there). But if you look at the junior facuilty members, there are 5 female professors out 18. So from the pipeline point of view, it is just a matter of time.
Unfortunately, that argument doesn't always work - hence the phrase "leaky pipeline". I don't know specifically for US maths professors, but in many situations where women are in a small minority a situation where the proportion of women goes down as you go up the hierarchy has persisted for many years. I remember hearing a talk about how US academia (in general, not specially maths) was particularly bad at this, even compared to academia elsewhere, because of the tenure track system - the time of life when your system forces people to put work above everything else in order to get tenure is the same time of life when mothers or those who want to be mothers find it hardest to do that. There were some pretty shocking graphs showing, for example, how much more likely a women was to make it to full professor if she was childless than if she had children, and how this did not apply to men. This was in one of the most famous US universities, but I forget which one.
