(Um, yep, that's the answer, stated plain. No need to laugh nervously.)
Miss Manners' response: "...you can merely state what it is, in fact, that you are knitting: 'Actually, this is going to be a ski mask. For skiing, not for robbing convenience stores.'"
So the conversation goes:
"When is the baby due?"
"Actually, this is going to be a ski mask. For skiing, not for robbing convenience stores."
"That's nice, dear, but I asked when the baby is due."
(Yes, Miss Manners, as you write: "...we all have the problem of inspiring strangers to voice the first silly association that comes into their heads.")
Thanks to Miss Manners' inability to understand a question and her unhelpful answer, we have an illustration of how a conversation should happen in fiction, a place where nothing should be easy for your characters.
Stories need complications, and having a character either not answer a question or answer a question with a question is an easy way to create one.
Such fictional conversations keep dialog going and can reveal character in both the questioner, who reacts to being stonewalled, and the stonewaller, by what he says to avoid answering. If you're writing a mystery, these conversations can provide clues.
How you resolve the conversation depends on what you need your story to do. If you need the information to send your character to another location or scene, you can have the answer eventually come out during the conversation. If you need your character to work a little harder, then use the need for information to send your character either back to ask the stonewaller again or forward to ask another character (who could also stonewall, which could prompt your character to wonder why all the secrecy).
We have a couple of examples from our Miss Manners illustration. First, Miss Manners herself doesn't answer the knitter's question. That shows us that she doesn't pay attention and that she thinks she's funny (she's not) and that she probably answered the question the way she did because she wanted to get a laugh, not be helpful. We all know people like that in real life, and they're maddening, aren't they? They make for good fiction, though.
In the case of the knitter, if she is pregnant and answers the question of when the baby is due by describing what she's knitting, we could wonder why she doesn't want to answer. Is she unhappy about her pregnancy? Or in denial? Or think that her due date is none of anyone's business?
Maybe she really is knitting a ski mask to rob a convenience store, but she's nervous about it and stumbles out this answer. Or maybe her friend the robber is sitting with her, waiting for her to finish knitting the mask, and the knitter is pleading for help. And when the stranger asks a second time about the baby, the knitter answers with the exact time and date of the robbery. (In a spy novel, you could use such a conversation to pass coded information.)
Return to some of the conversations in your novel and see if you can't complicate things with a little stonewalling.