It's difficult adjusting to the constant demands of a third party - or two, or three.
You're both exhausted all the time.
Her body may have changed very significantly, affecting self-image, affecting intimacy, affecting desires, affecting marriage happiness.
The kid won't eat.
The kid won't sleep.
The kid shits 30 seconds after you change her diaper.
The kid jerks his head as you're carrying him through a doorway and bangs his head and screams.
The kid is playing with you, looking you in the face, and all of a sudden lets loose with a wicked headbutt right to your kisser.

You'll wind up getting blamed for all kinds of shit that is just not your fault.

Add a little possible post-partum depression into the mix.

Yep, it will test your marriage, your character, your relationship.

Good luck. It gets better.