1) Use the terms "preggers" or "preggo". If you're over the age of 12, the term is pregnant, or knocked up if you feel especially crass. Adults who refer to themselves as being "preggers for the third time" sound like trash ghetto 18 year olds.
2) Compare my belly to other women's. I do that enough for both of us.
3) Ask me if I'm having twins. If I was, the polite thing to say would be "Wow! You don't look big enough!" no matter how huge I was. Unless you happen to be pregnant as well, and we have already established a long-standing "I'm bigger than you" dialogue, this is very inappropriate and, considering my fluctuating hormone levels, liable to get you smacked..
4) Lecture me about nutrition, or even just give me "that look" while I eat. I eat fresh fruit and fortified cereals so back off when I inhale a case of Nutty Bars in one day or have a cup of coffee. Oh, and the cereal is the kind marketed to adults, no cocoa fruity marshmallow stuff here.
5) Comment on how tacky the new tight maternity clothes trends are. I am facing being pregnant in August, for gods' sakes, and I am fully prepared to do my garden work in a crop top and low-rise shorts. If you don't like it, stay home in your air conditioning. As long as I don't accessorize with a cigarette and fur boots, I'm not as bad as Britney Spears and you survived that media blitz.
6) Ask me why I don't know what I'm having. Maybe it's because I don't believe in X raying Christmas gifts in November. And no, I don't care if that makes shopping harder for you. If you just have to buy me something, how about something I can use rather than some outfit the baby will only wear once. Buy me a breast pump -- they're gender neutral.
7) Ask me what names we have picked out and then criticize them.
8) Explain to me why formula is "just as good" as breast feeding. I've been through this before and even if I hadn't, I and all scientific medical evidence have to disagree with you on that. If it's what you need to believe to feel good about your parenting skills then fine, but I don't need to. Formula isn't toxic by any means, but even the Enfamil ads say that breast milk is better.
9) Ask me if I'm not just a little old to be "starting over". My daughter's only 9; it's not like we were going to be driving her up to college in the fall. And I'm only 31. Women are increasingly having kids in their forties now; it's not like the hospital staff will be torn between admitting me to either the maternity ward or geriatrics.
10) Blame all my moods on hormones. Hormones amplify moods; they don't create them. I might overreact to something, but there has to be something to set me off in the first place and if it's that you're being a butt, then my bursting into tears doesn't make you any less of a butt.
11) Ask me how much weight I've gained. Unless you're my doctor or you're pregnant too and comparing notes, my weight will never ever be any of your business. Period.