Yeah Julia Louis Dreyfus was pregnant during third season, it was hidden by having her either in baggier clothing or hiding the pregnant belly, in one instance sitting on the couch holding a pillow on her lap. Her absence at the start of Season Four was due to JLD's real-world maternity leave, in the show Elaine was out-of-the country with her current boyfriend, her psychologist, which oddly enough started a season-long arc with Crazy Joe Davola who fell off his meds since he couldn't get a refill while the psychologist was out of the country.
Shows can do it in crazy, fun, ways. But, "Ummm. She's getting fat," is an odd way to do it considering how "fat women" and "pregnant" women don't look that much alike and gain their weight in different ways.