Guess what? She knows she is getting bigger, and she probably feels huge. She doesn't need you to point out how big she is. Ask yourself, would you want someone walking up to you and saying "You're so big!"? Unless you are 5 years old or less, the answer is probably no.
"Are you sure you're not having twins?"
Asking if she is having twins, or saying something like "Are you sure there isn't another baby in there?" isn't likely to make her feel good. In fact, for most women it is pretty insulting. All pregnancies are different, and can be carried differently. Some women show much more than others. The only people who need to be worried about how many babies are in there are the mama, her partner, and her primary care provider.
"You look ready to pop!"
This is similar to telling a woman that she is huge. While always inconsiderate, this one is especially bad if you don't know how far into her pregnancy she is. Telling a woman who is only 7 months pregnant that she looks ready to pop is just rude and will likely just make her feel bad.
Some people don't mind this one, but I would strongly recommend that you be absolutely sure that someone is pregnant before asking. If the person just carries a little bit of belly fat, or has just had a baby, this is very insulting. Here's a tip... if you see a woman walking around with an infant, don't ask when she is due.
"Was this a planned pregnancy?"
This also applies to asking if the baby was a surprise or worse, a mistake. It is none of your business if she was trying to get pregnant or not. Regardless of if the pregnancy was planned or not, never refer to a baby as a mistake. Period. The end.
"Can I touch your belly?"
For some reason, people seem to think that just because a woman is growing a baby, her belly should somehow become public property. Would you like it if anyone, be it a friend, family member, or a random stranger on the street walked up to you and started rubbing your belly? Probably not, so you probably shouldn't do it to someone else. I encourage my clients who are offended by this to reach out and start rubbing the other person's belly to see how they like it. Asking first is a step in the right direction, but it can still put her on the spot and make her uncomfortable. Wait for her to offer. If she doesn't offer, just assume that she doesn't want you to.
"You can't eat/drink/do that!"
Once again, what another adult is doing is probably none of your business. In the United States, we are particularly bad about trying to tell women what they can and can't do. When it comes right down to it, that woman is an adult who can make her own decisions for herself and her child, and you have no business telling her that she can't. If you see a pregnant woman with a glass of wine, don't begin telling her off. It is her decision. When it comes to alcohol, many people believe that moderation is key, and that having a single drink now and then will do no harm. Ditto eating spicy food, running, and pretty much anything else you see a pregnant woman doing or consuming.
This is another one that a lot of women are fine with, as long as it is asked respectfully. Regardless of whether she is planning to become a stay-at-home mom or keep working, her decision should be respected, especially if you don't know her circumstances. A mother should not be shamed for continuing to work after a baby is born. It doesn't make her a bad mother. In fact, if the family needs the income, it is an example of being a good mother in supporting her child. A mother should also not be shamed for being a stay-at-home mom. Choosing to quit her job to focus on her children doesn't make her antifeminist. She is making the best decision for herself, her family, and their situation. Some women want to be stay-at-home moms, and that is fine. Some women want to or need to continue working, and that is fine too.
"You're planning to have a natural birth, right?"
This also applies to "You're planning to breastfeed, right?". The phrasing is the problem here. It would be perfectly fine to ask "Are you planning to have a natural birth?" or "Are you planning to breastfeed?", but in changing the wording, you are now shaming any mother who isn't planning to do what you are implying she should. Each mother is different and makes her own decisions for herself and her child. While it is fine to be curious, it is important to be open, and respect her decisions even if you don't agree with them. This is particularly true if you don't know her circumstances. There may be all sorts of factors in her life effecting her decisions, and she shouldn't have to endure your judgement.
Your horror stories
For some reason in our culture, and I have been guilty of this too, we are fascinated by sharing horror stories. Trust me, the mama you're talking to doesn't want to hear your horror story about the 75 hour labor that your sister's friend's cousin had. She certainly doesn't want to hear about how you knew someone who developed complications and had to be rushed to the hospital because she almost died. Chances are, she is already having worries of her own, and having you tell her about all of the things that you know of that may go wrong will just make it worse. Instead, try to be positive and share positive birth stories only unless she asks for some reason.
Any unsolicited advice
The keyword here being unsolicited. If a mama asks you what you did for heartburn when you were pregnant, by all means, share your words of wisdom, but if she didn't ask for your views on her sleeping habits, eating habits, bodily functions, etc. don't tell her what to do. I'm sure you mean well, but all pregnancies are different. Her pregnancy and symptoms may be completely different from what you experienced, and you constantly telling her what she should and shouldn't be doing is exhausting and can even be a little demeaning.