Are you there book lovers? It’s me, Margaret.

The one place where Angela Nguyen could spend unlimited time as a child growing up in Seattle was the library. She devoured books, especially coming-of-age stories. It was there, in the young adult section, where she encountered a book her middle school friends had been telling her about: “Are You There God? It’s Me, Margaret,” by Judy Blume.

“I didn’t know what it was before I read it. I thought it was going to be a religious book, to be honest. ... But I had friends who read it and said, ‘Oh, you should read this,’” says Dr. Nguyen, recalling schoolyard chatter from two decades ago. “‘It’s got girl stuff.’”

Judy Blume has been giving tween girls something to talk about ever since “Are You There God? It’s Me, Margaret” was published in 1970. Her iconic novel has been celebrated as a relatable guide for adolescent girls as much as it has been banned for its matter-of-fact discussions of menstruation. A new generation will get its chance to meet Margaret this month, when a big-screen version (rated PG-13) arrives April 28.

The cinematic debut is a win for screenwriter and director Kelly Fremon Craig, a fan of the book who was the first – after others had tried for decades – to secure Ms. Blume’s blessing.

“She wrote a story that feels universal and relatable to anybody, any decade,” Ms. Fremon Craig says in a Zoom interview. “When something is written with real honesty, that’s the effect it has.”

Fans also have a documentary about the author to look forward to: “Judy Blume Forever,” aimed at viewers age 16 and over, is available on Amazon Prime Video starting April 21.

A lot has changed since Margaret’s debut five decades ago. Nearly half of U.S. states have passed laws or set aside funding that supports access to free period products in school bathrooms. Reusable sanitary products have wider acceptance and availability in the marketplace. Female athletes debunk cultural myths about the menstrual cycle and performance. But turmoil still swirls around when children should be taught about their changing bodies. A recent Florida bill would prevent teaching about menstruation in elementary school, even though some girls can get their first period as young as 8. 

Ms. Blume’s book, written from the perspective of Margaret Simon, who is turning 12, has helped to fill the gap over the years on topics that adults can feel reluctant to address. The author has shifted the conversation around menstruation, say women’s public health experts, from something to hide or be ashamed of to something normal.

“In theory, everybody thinks parents are talking about [puberty] with their kids, but they’re really not,” says Marni Sommer, an adolescent health expert at Columbia University who has conducted research on three continents and co-authored “A Girl’s Guide to Puberty & Periods.” Ms. Blume’s writing talks about a sensitive topic “in a way that is confidence-building, and makes a young person feel normal,” Dr. Sommer adds.

“Are You There God? It’s Me, Margaret” is just one of Ms. Blume’s 29 books that combined have sold more than 90 million copies and been translated into about three dozen languages. The story centers on Margaret navigating a new school and new friends right at a time when she is feeling particularly self-conscious. Not sure what to do with her big feelings and questions, she shares her confusion directly with God. (She doesn’t identify with any particular religion, but has a Christian-raised mother and a father who is Jewish.) 

For Dr. Nguyen, the only daughter of Vietnamese parents, Ms. Blume’s books were a lifeline in the midst of middle school. 

“I grew up in a really strict household,” she says. “My mom never made me feel shame about getting my period, but it was not something we really talked about.”

Today she’s a postdoctoral researcher in menstrual health and education at Columbia.

Em Craig was only 9 years old when she sneaked “Are You There God? It’s Me, Margaret” out of her older sister’s room. Ms. Craig reveled in the story, taking in Margaret’s first forays with sanitary pads and a trip to the department store to buy her first bra.

“It’s like getting gossip that’s just too good to share. You don’t even know how to share it,” says Ms. Craig, a graduate student at UMass Boston. She was reluctant to talk about the book with her sister, Victoria – or mother, even though she had read and loved the book herself. “It was too embarrassing to go to my mom, and it was embarrassing to talk to my friends, so it was like the book was the only resource.”

That feeling is a fairly universal experience, explains clinical psychologist Lisa Damour, author of the recently released book “The Emotional Lives of Teenagers.” She says early adolescents often exhibit two key features: “One is the drive towards autonomy – kids want to be independent; they want to be separate. The other is the drive for privacy.” 

While Margaret may have turned to God for answers, scores of young readers turned to Ms. Blume, filling her mailbox with thousands of letters. In the 1980s those letters climbed to nearly 2,000 each month. Ms. Blume kept boxes of the missives, sometimes penning replies. She recently turned over the collection to Yale University to be housed in its archives. 

Ms. Fremon Craig was an adult when she wrote her first letter to Ms. Blume. She had recently finished directing the 2016 comedy-drama “The Edge of Seventeen,” for which she also wrote the screenplay. She wanted to adapt the Blume book she had loved as a child since first learning about it while splashing around in a friend’s pool. 

Ms. Blume agreed, and the director says working with the author was everything she hoped it would be. She is also quick to emphasize the importance of not only Margaret’s wondering if she’d ever fill out or kiss a boy, but also her spiritual journey. “I remember at that age, asking if there was something greater than us, if anyone was in charge,” she says. “You just want to know it’s going to be OK.”

During an interview on the “Today” show in January, Ms. Blume, who is a producer on the film, said she loved it. “How many authors of the book can say, I think that movie is better than the book?” she added. 

After a recent screening in Cambridge, Massachusetts, one young woman in the audience says she connected with “Are you there God?” immediately, even though she never read the book. 

“It was so realistic,” enthuses Sophia Perez, a songwriting major at Berklee College of Music in Boston, her ponytail of tight dark curls bobbing. She says she used to pray to God to help her grow taller. “It really appealed to the kid in you and brought back all of those awkward feelings.”

Em Craig’s mom, Lena, says she is looking forward to seeing the film with her two daughters. 

“Everyone has those questions as kids. Everyone has family problems, or has to do something uncomfortable like moving,” she says. “It’s all timeless. It’s all relevant.”