If there's anything we've learned in the past ten years, it's definitely to not comment on people's weight - and also not to mention people's pregnancies, unless they directly mention it to you. This means avoiding questions such as 'when is your baby due?' or 'is it a boy or a girl?' unless the person you've assumed is pregnant has brought up the fact they're expecting.
A woman took to Mumsnet to express her horror at the fact a woman had mentioned her pregnancy bump - but she explained that she's 55 and "one stone overweight", not pregnant. She was mortified that the woman had assumed she was pregnant, and asked whether she was being unreasonable to think the woman was rude.