I was thinking about this. Majority of info from:
Debra Messing on "Will & Grace"
In Season sixth of Will & Grace, Debra Messing was pregnant and didn't appear in 5 episodes. She wore muumuus and jumpsuits during season 6, it worked to cover it up early on except in earlier seasons she wore fitted clothes. They then tried sitting down and hiding her behind a table but that failed.
Jane Leeves in "Frasier"
When Jane Leeves was pregnant, the writing staff used it to use fat jokes, call it binge-eating for her character Daphne, having stressed from her relationship with Niles. It took three Cranes (Niles, Frasier and Martin) to lift her after she tripped. Daphne was later sent to fat camp to drop the pounds. Fortunately for Leeves' self-esteem, her second pregnancy was written into the plot.
Katey Sagal in "Married with Children"
In 1991 on Married... with Children, Katey Sagal had an unexpected pregnancy that was written into the show. However, she had to have an emergency C-section in her 7th month of pregnancy, ending in a stillbirth. The pregnancy on the show was then regarded as a "dream". The writers deliberately did not write Sagal's two later pregnancies into the show, opting instead to write off her absences in a subplot in which Peg is traveling the world to reunite her parents. In scenes where Peg was shown, Sagal had her midsection obscured, such as in a taxicab or at a craps table in Las Vegas, and was often seen or heard talking to family members over the phone.
Julia Louis-Dreyfus in "Seinfeld"
Due to Julia Louis-Dreyfus's off-screen pregnancy, her character had to spend the latter half of this season hiding her belly behind furniture and laundry baskets.