"Are You Having Twins?"
Never. Ever. Tell a pregnant woman (who, by most odds, is carrying only one teensy little embryo) that she appears to incubating an entire litter. Not only are you inadvertently comparing her to a four-legged creature, but you also are commenting (and not very subtly) on her girth.
You should also steer clear of this related statement: "I'll bet you're due any day now." It's not possible to judge a due date by simply eyeballing a pregnant belly, so don't take your chances. Sure, the woman in question may be due today or tomorrow, but if her baby's not scheduled to appear for another month or two, you can imagine how welcome your observation must be.