What did we expect at the What to Expect When You’re Expecting premiere? Nothing less than pretty off-the-runway looks from the cast’s leading ladies: Brooklyn Decker (in Giambattista Valli), Elizabeth Banks (in Alexander McQueen), Cameron Diaz (in Valentino), Jennifer Lopez (in Maria Lucia Hohan), and Anna Kendrick (in Elie Saab). As for working on a film about the pleasures—and pains—of pregnancy and becoming a mom, Banks said: “It was a great, really familiar thing.” (Banks is mom to son Felix.) So what’s her one mommy must-have? “Great help,” she said. “Grandma, for instance!” See more stars at last night’s hottest parties in the gallery.

— Caitlin Petreycik, with reporting by Andrea Simpson