Rule #1

You are always wrong. Trust me.

Pregnancy is hard work for Mums, and hormones can do unpredictable things - like claw Dad's eyes out when he choose the wrong time to assert he is right. If you disagree with Mum you still say "yes dear". The same hormones that would claw your eyes out will just as quickly give you a cuddle five minutes later, and that is much more important than being right. 

Rule #2

Mum knows best.

Discuss with Mum breast feeding, formula milk, sleeping, playing, feeding, and everything to do with the baby. Make sure to contribute the the conversations and don't just nod your head. 

But at the end of all of that, it is Mum's body and in most cases Mum is primary care-giver. When it comes to the baby, she outranks you.

That's okay, let her have that victory. We still outrank her when it comes to taking out the garbage, using the remote control and killing small animals.  

Rule #3

You need to grow up.

You know the idiotic things that you have done all your life? Mum used to think they were part of your roguish charm and they are part of the reason she fell for you in the first place. Be vary careful, because Pregnant Mum may now regard those things as a threat to the baby. The hormones mentioned in Rule #1 may be encountered in the circumstances too.