Are You There God? It’s Me, Margaret, the puberty bible and icon of YA literature, is, among other things, the story of an 11-year-old’s desperation to grow breasts. “We must, we must, we must increase our bust!” goes its most famous line. Rachel McAdams’ time on the set of the book’s new film adaptation is a testament to the fact that you don’t stop feeling weird about your boobs once they come in. Surprise! It’s a lifelong affliction.

“I felt like a milking machine,” McAdams says. She’d given birth to her daughter just five months before filming started and was pumping between shots. Then the costume designer threw her a curveball. “Why don’t we just try this without a bra?” McAdams, who plays Margaret’s mother, Barb, recalls her asking. “The great irony is Margaret just can’t wait to get into a bra. But I’m playing a kind of wild child, hippie artist mom [who] doesn’t wear a bra throughout the whole film.” Going braless while breastfeeding is no joke — the leaks, the size changes — but McAdams was drawn to the creative potential. “[We] really wanted Barb to feel like a real person who’s still figuring herself out, to put Margaret and her on similar trajectories.”