I had four children all after residency. First at 31, then 33, 36,39. We pushed it a bit for 39, it's true, but the point is you can still have a big family and not do it during residency. Personally I can't imagine having had the life I have with residency in terms of kid activities and busy-ness. I have a non physician husband. I worked full time until kid #2 as an attending and that was tough-had to stay in hospital some weekends, he brought her to breastfeed. Her babyhood is a big blur. I couldn't imagine doing that as a resident. PLus we have the bonus of having extra income for babysitters and such.
So I say, it would depend what type of residency you do. It can be done but you will need a ton of support, and be prepared to miss things they do. If you are ok with that, then go for it in school/residency.
I don't mean to be a downer, just wanted to add the waiting perspective. I give the ladies on here who did it a lot of credit, but I could not have.
Also you should know the stats for students matching who graduate school then wait a year to go into match are pretty poor. If you want that extra year, definitely do it by extending.