Blended family

So we have a blended family. Between my husband and I this will be our fifth child, but our first together. He has three from his first wife and I have a daughter. They all live with us other than the every other weekend his three visit their mom. Now that I’m showing and the pregnancy comes up in passing with people while we’re out, we’ve gotten the “you know what causes that don’t you”. Yesterday and today both someone said that. I know I should just laugh it off and I have been but I still think it’s just rude, especially when they don’t really know the circumstances. 🙄