I don't think it's so much the baby itself that strains marriage but other factors brought on by having children in general. I mean I totally fell in love with watching DH become a father and how wonderful he really is.
However, things like sleep deprivation makes people very nasty to each other. If you run into any money struggles that can be difficult of course. Also, the lack of time you now have for each other puts a damper on things.
Honestly, with my first things were fine. It wasn't until we had DS2 that we really had to start working at our marriage.
I think going into a realistic mind set helps. Have some discussions on how you can work as a team to care for the baby. How can you both still get some alone time with friends and alone time with each other? How will you handle financial changes that pop up? How will you handle family members input into your parenting etc... It will take some time for you all to figure out how to fit into your new roles.
I mean you can't predict every situation or plan for everything but keeping open communication is very important.
We go through periods where we are very close and then periods where I feel like he is just my children's father. It's a strange roller coaster but being able to pin point when you don't feel happy with each other and being able to get back on course will make a world of difference.
Good luck and don't stress too much about stuff that hasn't happened yet and try not to judge other couples because you never know what goes on behind closed doors!!