I would look at it from the fairly selfish perspective of would you be willing to give it all up (the postgrad) if something happened with your child that meant you needed to care for them and put your studies to one side? I know that is a fairly selfish way of looking at it but I know that if I were to have a child in the next few years (I'm almost 22, graduate in the summer and am applying for postgrad courses at the moment) as well as studying, I can't help but feel I wouldn't have a bit of resentment that I hadn't waited if something happened in my family situation which meant I had to totally stop studying and potentially never finish.
I know this can happen with a job too but I feel that because you obviously get some financial pay back for the time/effort/money you invest in an actual job, if I were to give it all up due to my child it wouldn't feel as though I'd "wasted" a couple of years on study. But if I were two years into a three year course, I wouldn't really get much return for it beyond more debt as if I never finished it I'm not sure what good a half finished course would do in terms of enhancing my life but if I were to say, go back into the working world (say, only working a few hours a week or something low key) a few years later I'd be able to transfer the skills etc from the job I'd had in the two years much more easily than if I were trying to transfer academic knowledge.
Obviously this is all my own view point though and you're welcome to take it or leave it, but I see having a child as something which I'll only do when I'm willing to give up my entire life for it should that need to happen, and if I were considering further study (as I am now) which I know will commit me for the next few years, I wouldn't also consider having a child in that period simply because I know that if I only finished half of it, I'd regret not finishing it and having a child later. Obviously, I'd love my baby unconditionally and would give up studying for it, but I think I'd feel more resentment of the fact I'd done that than if I'd given up a job for my child.