'What to Expect When You're Expecting'? Cameron Diaz's baby bump

Cameron Diaz at the "What to Expect When You're Expecting" premiere Monday at Grauman's Chinese Theatre in Hollywood.

The new comedy “What to Expect When You’re Expecting” promises two sure things moviegoers can expect: lots of babies and lots of pregnant women.

Babies and baby bumps were less common, however, at the movie’s black carpet premiere Monday at Grauman’s Chinese Theatre, though there were appearances from a pregnantMelissa Joan Hart, expecting her third child, and the twins who played the baby adopted by Jennifer Lopez’s character in the film.

Among the leading cast members, Lopez alone has given birth in real life, but the production gave her costars a taste of the pregnancy experience — though for some it was less than realistic.

Brooklyn Decker’s character, for example, has a stress-free, pain-free pregnancy that wins her the title “magical pregnancy unicorn” from costar Rebel Wilson’s character.

“When I put on [the prosthetic belly] they were like, ‘OK, we need it thinner, and we need it more bronze. Tan the belly. It has to look perfect,’ ” Decker recalled.

“I was like, ‘This is not fair! This is not what pregnancy looks like.’ I’m setting a horrible standard for myself.”