I like to think that I had a healthy respect for the significant changes that a baby would bring into my life. We can all guess what weeks of sleep deprivation and undernourishment feels like, but until you're in the middle of it and holding a crying infant, it's hard to grasp. The same goes for the ways a partnership is affected by the arrival of a baby. There are things you can only learn about your marriage after you have a baby (aside from the obvious, like how your partner functions as a parent).
My spouse and I had been together for nearly ten years before we welcomed our son. We met as students, so those years included many transitions and moves and job changes and various other milestones that adults tend to reach in their 20s and once they're "established" (whatever that means). So, I definitely felt like — and still feel like, if I'm being honest — I know my partner pretty well. Still, there are sides of him (and sides of me) that didn't come out until our son arrived and we were officially parents.
To be fair, these sides had no reason to come out before we were responsible for another human being (even when we got a puppy, because it's not the same thing). Still, while we knew we were in for an entirely new experience, we didn't realize we would realize the following until, well, we did.