I think the truth is always positive to those who seek it. Knowledge protects - and protection and personal empowerment can never be a bad thing (except to those who prefer their sacred cows, but they're not being objective). I think your wife knows what she looks like pregnant because she can see herself in the mirror. She probably just wants reassurance that she's still beautiful or attractive to you, so she's asking for your subjective opinion. In essence, she's asking "do you find me gross like this"? And you could say "you don't look fat, you look pregnant, and it looks beautiful on you!" then make sure to give her a hug and a kiss or something so she doesn't doubt your sincerity. That way you aren't lying to her, but still giving her the reassurance she needs to be ok with what she sees in the mirror.