I've been asked that quite a few times by the elderly in nursing homes I've worked in...I just smiled and said, "Nope, I'm just fat."
My niece has pancreatic cancer that is cycling down into the last stages, which has her rail thin but with a distended abdomen. The most upset I've seen her throughout this whole thing is when the other day someone asked her when her baby was due. You'd think she'd be more upset over her terminal condition, but nope...was very emotional because her belly is big and they can't do anything about it.
It's safer just to speculate but never, ever ask. If people want you to know they are pregnant, they'll either tell you or wear a shirt saying they are, but I NEVER ask.
Poor, NYboy!