There is what is referred to as the “leaky pipeline”. There are more women getting bachelor's degrees than there are getting master's degrees. We then lose some at the transition from master's to doctorate. We lose some more from doctorate to post-doctorate, and more still from post-doctorate to university professors. The ratio of women in the natural sciences and engineering faculties is about 22% women to men.
On March 28th, 2012. See this statement in context.