i had dd when i was 36, and had only recently met and married my dh (i was 35, he 8 years younger!) i have two master's degrees and have been working in my field (switching paths kind of in the middle) for around 10 years. i honestly would not want to have a child without a partner, personally. unless you have awesome parent-friends and strong family support, i guess. it is hard, harder than i would have imagined, for just the two of us to raise our 1 dd, and i don't know how other folks do it.
career-wise, i have friends who are married & completing phd programs while parenting young children , and for them, it is very difficult. my own dh remembers his dad getting his graduate degrees and telling him tearfully that he couldn't play at that time, he had to study,etc. i would not want to be in a phd program AND have a young child/baby. it's too much, honestly, and anybody doing it has my respect.
if i had my druthers, i would have timed my own differently, though. i'm at an age and length of experience where i could be moving up a little, (i'm in academia.. those next steps like dept chair/dean/director, etc) but because i have a young child & am pregnant that won't happen. discrimination like that is illegal, but it happens. every time. so at this point, i am a little thwarted career-wise (which is for the best, i don't really want to spend long hours at work rather than home).
i'd say, based on my experience and from what i've seen, the best baby-timing, career wise, if such a thing is possible, is after the graduate degree and after finding a comfortable place of employment where you intend to stay, and have worked there for at least 3-4 months (because you have to work somewhere for a year to have FMLA guaranteed to you). i wouldn't think working a few years then having children is more helpful-- ime it's just led to a lack of mobility. (and again, there's no way i would even think about doing it as a phd student.. in my field a master's is the terminal degree, and i would probably be ok with doing a master's and not working and a child.. but not much more than that).
eta: carma is right about fertility.. that's another thing to contemplate. no guarantee it's going to work the first few times you try. also.. i feel in that regard i waited too long for comfort, and for having multiple closely spaced children. oh, well, you get what you get. :)