Recent research published in Science by Deborah Kaminski and Cheryl Geisler
about retention of STEM women faculty seems to suggest that women and men are retained and promoted at similar rates in most STEM disciplines, except for mathematics and math intensive fields. They found since 1990, from time of hire in science and engineering fields, median time to exit is about 11 years for women and men. In mathematics, 50% of faculty (male or female) depart their positions earlier than other disciplines, but women leave math departments even sooner (~4.5 yrs) than their male counterparts (~7.3 yrs). Certainly, some fields are hiring a smaller percentage of women than men; fewer women exist in the pools, resulting in fewer women across academic ranks. More research into the causes of exit and the utility of time to exit may be needed, but it is clear that continued investigation into the practices and outcomes of recruitment, hiring, and retention of women in STEM is important.
Then, read the Sigma Xi article in American Scientist by Wendy Williams and Stephen Ceci about the evidence dismissing the usual factors for under representation of women in math-intensive fields (ability differences; occupational and lifestyle preferences; and sex discrimination), and understand the issues related to motherhood and the systemic policies in place that do not historically support concurrent career and biological timelines, especially for math intensive disciplines.
You can also listen to the podcast or read the transcript of Survival Analysis of Faculty Retention in Science and Engineering by Gender, Deborah Kaminski and Cheryl Geisler, Science, 17 February 2012: 864-866.
You may read more about the study by Williams and Ceci from the Cornell Chronicle.