It is hard to say because a lot of other things have changed (jobs, careers, where we live) during that time as well. So it is hard to pin down what causes which change. Plus, we had only been together 2 years when we had our first, so how we were pre-kids was very much the honeymoon stage and some of what we've been through a relationship can go through with or without kids.
I will say though, the first 12 months put huge strain on our relationship. Not just with our first but with our 2nd 5 years later as well. It is just a full on stage where you are often left shaking your head saying "will things ever be normal again?".