Early in pregnancy, most women gain weight and feel bloated enough to lose their waistlines, but not so much that there's a baby bump that's obvious to others. This may be especially true for plus-size moms-to-be, who usually take longer to start showing that typical, rounded baby belly than smaller-framed women.
The best thing you can do is spread the news when you're ready -- letting your family, friends, and coworkers know you're expecting eliminates any doubt or guesswork on their part. (After all, most people are cautious to congratulate a woman on her pregnancy unless they're pretty darn sure there's a baby in there.) Try slipping your pregnancy news into everyday chitchat, or ask people for advice like, "How did you find your nanny? We need to start thinking about childcare for after the baby's born." Standing with your hand on your belly (or rubbing or patting it) is also a pretty obvious hint that you’re expecting.
Another obvious solution is to buy a couple of cute T-shirts that announce your pregnancy. You can also start wearing maternity clothes or empire-waist blouses. The cut almost guarantees that even the slightest belly bulge will be noticed as a baby in the works.