It's well known that a new baby can create a lot of stress on the parents' relationship with eachother. However, if you anticipate this and have a plan for dealing with it, it'll be easier to work through and help you thrive as a couple and family.
1. Expect that the first year of new parenthood will strain your relationship.
Any type of role transition tends to be stressful. When you add pregnancy hormones into the mix, and you're getting to know the personality of your brand new person, this creates quite an intense cocktail of changes that you'll need time and mental space to settle into.
If your vision of what new parenthood "should" be like and the reality are starkly different, then that itself can be stressful. If you're tired, you may end up ruminating about why you and your co-parent are experiencing relationship stress. It may be more useful to think of relationship strain as simply par for the course for many new parents.
A new baby tends to create a wider ripple effect on family dynamics than people expect. Becoming a parent can change your relationship with your own parents, your in-laws, and even your siblings. It can make you see your relationship with your own parents through a different light, or your parents may become more intensely involved in your life because they have love and advice to give related to their new grandchild.
The same applies for sibling relationships. You may end up having more involvement with your siblings, especially if they also have children. And, siblings sometimes react to changes they observe in your relationship with your shared parent/s. For example, a sibling who doesn't have children may become envious of the extra interest their parents are showing in the family that has the new baby.
The takeaway here is that a new baby, and especially a first baby, creates lots of changes in roles and dynamics within couple relationships and extended families. Give yourself as much kindness and breathing space as you can while everyone is coping and adjusting.
2. Take the attitude that you'll be discovering your individual and combined strengths as parents.
It's pretty obvious to say that parenthood will likely match your expectations in some respects and surprise you in others. While there may be aspects of parenting that you find harder than you expected, you'll also discover that you have strengths as a parent that you perhaps weren't anticipating, and so does your partner.
Also, pay attention to relationship strengths that emerge/develop. It's easy to focus on the negative (e.g., being increasingly impatient with each other). Therefore, make sure you deliberately also focus on the ways you make a good team as parents.
3. Does it seem like your partner has changed a lot as a result of becoming a parent?
Your partner will mostly still be the same person before/after becoming a Mom or Dad. Sometimes people catastrophize if it seems like their partner is more interested in the baby than in them. Other times people feel increasingly irritated by aspects of their partner, even though those characteristics were present in their partner before. For example, if your partner tends towards being selfish or controlling, you might find that you feel especially annoyed and upset by those aspects of their nature when you have a new baby. Partly this is because having a baby together tends to intensify a sense that you are "stuck with each other," and that sense can create anxiety.
Whatever reactions you find yourself having to your partner, try not to panic or ruminate. Give yourselves time to find a new rhythm as a couple. If you need help to address ways in which you're frustrated with each other, you can always see a couple's therapist to do that.
4. Remind yourselves that you're both doing your best.
There is lots of great advice for parents. However, actually implementing is harder than it sounds. You may have had plans about what type of parents you ideally wanted to be but then find that reality is not quite matching your ideals.
Provided you are reasonably conscientious people, you'll likely do better if you're nice to yourselves than by being excessively critical of yourselves or each other.
5. Use your parental leave options.
Since there is a lot to adjust to for new parents, do yourselves as a favor and use whatever parental leave options you have to the fullest extent possible. Minimizing work stress will allow you to bond and have the energy to work out any issues as they emerge. If the non-birthing parent can take some time off work, it'll be much easier for them to support the birthing parent, establish their relationship with their child, and learn about their own strengths as a parent.