:( I was in a similar boat. Still am, I suppose. I am 23 weeks pregnant with our 4th. All were planned. Their ages are 4 1/2, 3, and almost 2. Now, we'll have another in early June.
My in-laws only had my husband and his brother. Even though they both come from families with 5 kids, they think 2 is plenty....and they certainly let me know. :) I have 7 siblings, so I am used to large families. I never intended on having more than 2 children, but I honestly LOVE being pregnant and LOVE having my kids. Never second-guessed that!
My mother-in-law is very opinionated on how many kids SHE thinks WE should have. My FIL is similar, but not so much to me as to my husband. It hurts. It really does. We have never asked them for any kind of support financially; we don't need to. We don't ask them to babysit unless it's a last resort because we know how they feel and (to be honest) I'm still bitter that they think they get to decide how many kids we should have.
I don't think you're being too sensitive at all. Those comments ("oh, you're having another?" or "ever heard of birth control?") are funny like the first 2 times; they are hurtful after that. I don't think my in-laws get it. I know they know how hard I work at raising the girls, and I know they're excited we're finally having a boy. BUT, they don't express positive things about the number of kids....even when we're in public and in front of strangers. "These are my grandkids...yes, ALL of them." I mean, really!?! 4 is not really that many.
And of course the hormones don't help! :) All I can say is try to laugh it off with them. Maybe they just don't get how much it hurts your feelings? I don't think my in-laws get it. They are having to help out my brother-in-law a lot financially and otherwise, so they give him their opinion all the time on how he should live....I can only assume they think they need to do the same with us (even though they've never supported us financially or have been asked for anything from us!!).
Try to just enjoy the pregnancy with your husband and kids. Hopefully, you don't have to interact with them too often during the rest of your pregnancy! :) Hang in there!