May I add some I have recently encountered:
Unless you are the OB, do NOT argue with a mother over stuff just because you read an article on it. I have a friend who keeps doing this to me, and she's never been pregnant.
If the mom says no baby shower, don't tell her she's having one whether she wants one or not and that it's not HER choice. Um yes it is.
Don't look up every single baby item on the baby registry and inform the mom of all the negative reviews it got. I went through literally 30 different infant seats because someone kept telling me every single one had awful reviews for safety and ease of use. I'm sorry, I can't afford a $300 infant seat from top brand.
If the mom says she wants to breastfeed, don't insist on buying her bottles and do NOT sign her up for formula stuff!!!!!!! All of a sudden I started getting samples of Enfamil....I never signed up with them. Turns out a friend signed me up because I might "change my mind.". I gave the cans to a neighbor who does formula feed. I don't want the temptation.
And I second calling the mother-to-be Mama. It drives me absolutely insane. I HATE IT!!!! My son calls me mommy, and my dh refers to me as mommy when he is talking to ds, that is it. I HATE it when I answer the phone and I hear "heya mama!". *shudder*.
When a woman asks "Do I look pregnant or just bloated?"....don't say "You don't look pregnant.". Bad idea, bad move.
When a woman is obviously enormously pregnant and hopping from foot to foot in line in the bathroom, you don't have to do this, but it would be nice if people let them move up. Babies tend to bounce on the largest organ near them, and a full bladder is just that. We aren't entitled, but it's nice to be offered.
If you are the coach, you do not get to dictate the birth plan. I might say no pain meds today, but when I am in labor, that could all change and don't you dare tell me "Remember, you said no pain meds.". Your job is to support me in whatever choice I make. If I start begging for an epi, don't you dare argue with me.
If you are in the room when the baby is born, you do not get to hold the baby before the parents and new siblings do. Sorry, but no. We are kicking everyone out so ds can have an hour or so to bond with the new baby and we can bond as a new family. And don't blow off the other siblings. Just because there is a new baby doesn't mean you get the back burner them.