Of course it is! Think about it, if a woman isn’t pregnant and you ask if she is, the implication is that she is overweight. In much of America (can’t speak for other places), if a woman is overweight, the only acceptable reason is pregnancy. So, if you ask that question, you’re implying that she is overweight and therefore, unacceptable. You might be genuinely curious, but at that point, it doesn’t matter. You have already raised the specter of “fat” in her mind.
A word to the wise – don’t ever ask that question unless you can relate your alleged curiosity to comments she has made about her own morning sickness, hiring a nanny, buying a crib for her new baby, etc. If you are dense enough to ask, and she says, “Why? Do I look fat?” you can say, “No! You look just right. I heard you talking about finding a daycare for the new baby and wondered if you were expecting.” But really, just don’t ask. It’s not your business and she’ll tell you if she wants you to know.