Approach toward parenting
Raising children together can be one of the biggest blessing as well as a litmus test for a relationship. First of all, these tiny humans can take up so much space in your life and mind that your bond with your SO takes a back seat. To top it, if you disagree on parenting styles and what values to inculcate in your children, it can be a recipe for disaster.
You end up making parenting mistakes and blame each other for that. Discuss these things before you take the plunge into parenthood and you can dodge a major bullet.