Discussions of work/life balance, timing your procreation, and parenthood are common and recurring themes in the science blogosphere. My advisor happens to be extremely supportive of his grad students (men and women alike) having kids during grad school. Herb would argue that best time is during grad school. He even went so far as to boast to the new cohort of grad students that he has the "most fertile lab in the department" since some abnormally high percentage of his previous and current students have had one or more babies during their grad student tenure. His general rule of thumb, having watched many of his grad students have kids, is that each baby adds about a year to your Ph.D. My office mate took about 7 or 8 years to finish but had two kids during that time so that puts her right on track in terms of her progress.
You might think that Herb had his child while in grad school. Quite the contrary. At the celebration dinner after my office mate's defense, Herb mentioned in conversation that he took his son to museums 4 days per week when he was a toddler/preschooler so that his wife could get her science done. Confused at how this was possible, I asked for clarification how he spent four weekdays at a museum all day entertaining his son. His response? He was already a full professor so he had the flexibility. He was a full professor when his son was born- he was no spring chicken. For those of you who are less familiar with the traditional ranks of academia, a person typically does not become a full professor until after they have completed their Ph.D., typically had a postdoc (~1-4 years), landed a tenue-track position as an Assistant Professor (5-6 years), gotten tenure and become an Associate Professor (probably at least 5 years), and then finally made the (typically) last leap in rank to become a Professor. That's minimally about 11 years after completing your Ph.D. Assuming one starts a Ph.D. at age 23 (young) and finishes at age 28 (fast), then you would be at least 39 before you become full professor. What this boils down to is that most women would be in less-than-ideal circumstances to start having kids when they are a full professor. My advisor had the flexibility in his schedule (and the reduction in pressure) that comes with the job/financial security of a full professorship. Interesting, no?