Adventures in Ethics and Science gives marvelously informative detail about the hazards and joys of bearing children while still in graduate school, with more narrative from readers in the comments section.
The worry that she emphasizes again and again is the need for good childcare, the expense of good childcare, and the inadequacy of even good childcare when the child is sick. I can't imagine who you can call--other than retired grandparents--for back-up childcare in that case. The only solution I know is for one parent or the other to take off from work. Skipping class is a hard thing for an academic to do. Babies do get sick, sometimes for a week at a time.
I've heard colleagues express similar concerns about whether it's possible to have a baby before tenure. GonePublic blogged about this some time ago, and commenters cited evidence that women with children fall behind in rank. Still, we can pursue our careers for 25 years longer than we can pursue reproduction. Reproduction is not for everyone, but in a better world women desiring maternity would not have such worries. And mothers would have the option to work outside the home--or not, as they wished.