A lot of us have first-hand experience with being mothers on the tenure track (and/or in grad school), and we all have unique experiences. The sets of circumstances we all face makes it difficult to compare across disciplines and universities. A lot of commenters on these posts have offered suggestions about why women in academia have lower birth rates than our male counterparts and a lot of really interesting discussion has been generated. However, I think it's impossible to generalize, or point to specific reasons, because I think there are a lot of different reasons out there. I often wonder if I would have ever had any children if I hadn't accidentally gotten pregnant during my PhD. It wasn't until getting tenure that I felt I could seriously consider having another one (and that seems even less likely to happen now). I know academic women who do not want children at all and I would imagine that they all have different reasons. In my own department of about 15 faculty, there are two married men without children, too.
I think that it's important and useful to discuss issues of family and gender-equality in academia without people taking sides about whether academics have a tougher time, or other professions are less family-friendly, or whether children are a life-style choice, or whether personal obligations of other kinds should be viewed in the same light. Mommy/Prof's point about the two-body problem is also extremely relevant to this discussion.
I'm not sure that I have anything different to add other than my own experiences and struggles. I sit here wondering if I will add to the statistics presented in the IHE story, by stopping at one child. My family has clearly been shaped by my career, but I hope it has not been stifled by it. H and I knew that we were ridiculously lucky to land tenure-track jobs at the same university and were afraid to hinder our research and teaching (before tenure) by having another baby. D was a toddler when we came here- not the independent almost 9-year-old that she is now and it seemed impossible to consider starting over again with another baby. I'm the optimist in the family and think that things would still have turned out fine if we'd had another baby, but obviously things would have been more challenging. I guess it boils down to the same issues regardless of your career and for women it results in making hard choices a lot more often than it does for men*. I don't have any answers about this, but I would be willing to bet that these issues will not be going away any time soon.
*OK, male readers- I know there are enlightened men out there who let their careers take a back seat to their wives', but you are certainly in the minority!