First of all, it's none of their damn business and I would LOVE to reply that way to all who ask. But more so, its a personal and intrusive question and I don't think I owe anyone any answers? nor do I think they have the right to ask it. (Yes, I know, free speech is in the Constitution! LOL)
I generally prefer the response of "never!!" to that question because people get weirded out and pissed and shut up immediately. Then I can walk away laughing or smiling or change the subject if I still have to talk to the offender. (They NEVER ask again.) To a small minority of more dense people, I follow that up with, "that is a personal subject which is none of your business". Then they seem to drop it.
The most recent offender was my brother. He and his wife keep his side of the family at an arms length from them and their 2 young children. They are very cold with me and my parents. It is what it is. I'm not harping on that? but then the ? from my brother (to my husband) then bugs me even more.
Thing is, because I don't trust my brother, I can't stand their circle of gossipy friends, and cannot help wondering why he even asked and how they will use this against us. (thankfully, they live kind of far away, so I don't see them regularly)
My husband thinks it was an innocent question which indicates the potential to patch things up in the future.
Meanwhile, IF we ever have kids I do not plan on telling them anything? ever. They can find out whatever future news we have through the grapevine? Probably sounds incredibly immature from the outside, but its really just a reflection of how much I don't trust them. (For one reason, they used to spy on me when we lived in the same town in the past, and have used other methods of spying since they moved away.)