I can identify with you! Not the exact same situation, but my mom also shows her love by giving copious "constructive criticism". Her "helpful" words often put a damper on otherwise happy events and can reduce me to tears like a little girl, even at age 37. Sometimes they can even say really inappropriate things that really ruin how I feel about something.
It's taken me a long time to realize that my parents will NEVER be those loving, 100% supportive parents often seen on TV. They don't know everything, and they are only trying to live their lives and love their children the best way they know how. Probably the only way they know how. It took me years to figure out how to distance myself from them emotionally - I don't mean to talk to them less, see them less, or love them less. But just to learn that it's okay for you to have different opinions from your parents. And you don't have to feel guilty about it. IT'S OKAY! Even though they may make you feel that their ways are "right" and whatever you're doing is "wrong", rationally we all know that life is not so black and white.
It's certainly your perogative to keep the pregnancy from them until the child is born, but that may really hurt them. And it's something you can't take back. If and when you decide to tell your parents about your pregnancy, I would have your husband present to support you. Psych yourself up before to have a strong, confident, and positive attitude. Start by telling them how truly happy you both are that you're finally pregnant again, and that you're delighted to be able to share this fabulous news with them. If they start to come in with something negative. both you and your husband should politely stop them immediately. Tell them exactly what you told us, that you're so looking forward to this baby that you don't want to hear anything negative. Sometimes I joke with my mom (something I wasn't able to do when I was younger) so that when she opens her mouth, I ask laughingly, "Hey, are you going to say something positive? Remember, don't be mean! The baby can hear you!" I don't know if that would work with your mom, but sometimes it helps to lighten the mood. And when they realize how determined you are in your position, they may give up trying to "help" you with their advice and opinions.
Finally, if you have a sibling or an aunt who's close to your mom, it may be helpful to enlist their involvement. Have them talk to your mom just before you share your news - have them tell your parents, not your news, but just that you have something to share with them and that you need them to be very happy and positive for you. This doesn't always work with my parents either, but sometimes it has.
Good luck and congratulations! You sound like you really know yourself, what's right for you, and what makes you happy. So I think you just have to keep trying not to let anyone else's opinions, even your parents, hurt you or affect how your feel. (I know, easier said than done!) But serious, the more confidence you are, the more likely they'll back off. If you sound unsure, they'll certainly feel the need to step in and put in their 2 cents.
Reminds me of my parents. They raised me to be career-focused. When I was preparing for my wedding, my mother would talk about divorce until I finally confronted her and told her to stop. I still find it hard living my own life and making my own choices - and dealing with their lack of support. My husband is very supportive and we have are friends with a couple in a similar situation. Now my husband and I are talking about having a child, and I'm dreading my parents' reaction. We are paving the way in our conversations. Even with preparing them, I am still a little nervous that my mother will talk about abortion. So the plan is for both of us to tell my parents together and then, later the same day or the next day, tell his parents. We expect the second set of parents to be much more receptive and to make up for the cold response from my own family. I hope I can help both Dreamy and other people with similar situations. It's reassuring to see I'm not the only one.