When they can't tell if you are fat or pregnant

How would you reply (no, smile and ignore doesn't count as a reply) when your friend/colleague/relative comments "Ohhh no wonder!" to you telling them you are pregnant? You would know then that they've been looking at your ambiguous tummy/flabby arms/chubbier face wondering if it's from the weight you've recently gained. They don't know and can't see any of your other pregnancy symptoms so they just think you've gotten fatter. Feels kinda annoying that they didn't choose to just congratulate you but instead make that "oh no wonder" comment isn't it? ☹ Wished there's something smart we can reply to that.