Perhaps you're doing well at work? Having a child now may get in the way of a promotion. Better to wait until after the promotion.
Did you just get promoted? Having a child now would be a really bad idea, you need to be focused on your job until you've gotten on top of your new responsibilities.
The economy is doing poorly, or perhaps you're not doing very well at work? It would be really unfortunate if you had a child, and then got fired.
Perhaps you need to lose a few pounds before getting pregnant?
Perhaps you've always wanted to run a marathon, and you think it will be harder get in shape for it if you get pregnant first.
Anytime you can wait for things to be just right before taking an action, that action can be put off indefinitely, because things will never be just right.