I know there's more, but the numbers are still far from even in many hard science & engineering degrees. I started out in Civil Engineering and even there, the ratio of guys to girls was scarcely better than when I transferred to Computer Science (this was in the few degree specific courses I took as well).

I do consider Bio and Medicine to be hard sciences and you are correct the gender imbalance is significantly less than some other science fields. However, it seems to be more acceptable in American Culture for parents to accept their daughters going into those types of degrees perhaps due to the relation to traditional career paths women have gone into (such as nursing). If that's true, it's rather sad and hopefully such preconceived notions die out in the near future.

I'm curious though as to how many female students end up going into Computer Science or Engineering because a family member or parent was in one of those fields. I have a few female friends either pursuing or have obtained a degree in Computer Science and each had at least one parent that was in a related field.