I would definitely do a search and check out the non-trad forum -- there are tons of threads like this on there. And over in that land, the general consensus is that it is far easier to have a kid
before med school than during residency. I mean, think about it...you have the two academic years which are much easier to anticipate in terms of scheduling and planning than your two clinical years...not to mention what kind of timing you'll have to deal with during residency, when you'll be on-call and doing other wacky shift-type finagling. And the scheduling is far easier to deal during M1/M2 if you're breastfeeding than during the clinical rotations of M3/M4 or residency, when you can be running all over the place...I mean, do you think you'll feel comfortable asking your attending if you can take 20min to go pump every 4-6 hours?
I also highly recommend that you check out the child-care policies of the schools that you wish to attend, as well as the policies of the schools (or future workplaces) of your fiancé, and then again when you're going through match. Some residencies will make it easy to have a kid, some won't...so you're better off doing research now while the child is still hypothetical.
Hmmm...as an aside, I just noticed from another thread or two that you don't seem to be too big on searching for things, so here you go:
Baby before or after med school?
http://forums.studentdoctor.net/showthread.php?t=252540
http://forums.studentdoctor.net/showthread.php?t=58503
http://forums.studentdoctor.net/showthread.php?t=214693
And no, I'm not trying to start a flame war with a statement like that, but it seems like you are new to internet forums. It's easy for many people to use the anonymity to be as callous, nasty and otherwise unhelpful as they can be since there cannot be any retaliation other than being banned...I mean, check out the post by the guy who was ranting about welfare mothers...lol, troll!
Basically, search first and you will always win.