This is total B.S.
I researched this concept several years ago using U.S. Census data and found that even in fields that are traditionally dominated by women (no credible anti-woman bias) The ratio of women to men steadily decreases with advancing degrees. For example, if a particular field features 85 pct. women to 15 pct men at an undergraduate level, it would then show something like 55 pct/45 pct at the Masters level, and maybe 30 pct/70 pct at PhD. - And this in fields like health care, education and library science which have *many* more women than men at the entry level. It's not bias, it's biology!
In the hard sciences, advanced degrees are often essential and there are generally fewer women than men even at the entry level. So naturally, there are going to be much fewer women with advanced degrees in the hard sciences.
Yup and most women do not pursue an advanced degree because they have families and are no longer interested in that kind of stress/etc. And I think women just aren't as inclined to "prove themselves" in the working world. Biologically speaking, we aren't meant to be the breadwinners. I say this from my experience as a female in a math/science field dominated by men.