The first year with a kid can be hard. My wife and I were in the worst spot of our marriage about 9-12 months after my first was born. I don't think it was ever really super serious, and we worked through it pretty quickly, but things took a pretty sharp downturn fast.
There were a lot of contributing factors, and some of them were out of our control. However, we really worked on those that were in our control and got things back on track a few months later. The biggest thing was me really cutting the work travel back so I could spend more time with her and my oldest son.
I don't think the kid is the cause of the issues. The child is simply an amplifier for things that you don't agree on. If you aren't on the same page financially, politically, religiously, etc. it can cause a huge issue because it always goes back to, "Is that how you want to raise our child?"
In all honesty, and I hate saying this (but we say it to each other), but my wife's cancer played a huge part in getting us to our current point, which is the strongest we've ever been. Going through something like that, with young kids, and not a lot of close family and friends living near you forces you to rely on each other. We spent the better part of a year team-working everything. She even said a few months ago that she never really could have known how much I loved her until I supported her through that whole thing.
Aside from my kids, my wife is my favorite person in the world. I adore the crap out of her.