Last month, dressed in a vintage red velvet tuxedo, Gabrielle Blair took the stage at Alt Summit, a design conference at Chelsea Piers in New York City. Her message: Don’t send a woman to do a man’s work — the labor of preventing unwanted pregnancies.

Ms. Blair, 48, was one of the original mom bloggers in the early aughts. In 2019, she appeared at Alt, which she co-founded, to give a talk on “connectivity, learning about yourself and your business.” This time, she told the audience: “I just want men to ejaculate responsibly. This is a very little ask. This is not hard to do.”

Ms. Blair was invoking the title of her new book, “Ejaculate Responsibly: A Whole New Way to Think About Abortion,” which debuted this week at No. 2 on The New York Times’s paperback nonfiction best-seller list. With its publication, she has added reproductive rights activist to her bio, which also includes Mormon, mother of six and home improvement expert.

The book, which some reviewers have called a manifesto and a treatise, is a clear departure from the work Ms. Blair’s devoted audience had come to expect. Her blog, Design Mom, launched in 2006, and it became a huge success as well as her family’s primary source of income. Time magazine named it the top parenting website of 2010, and the Blair family was featured on HGTV’s House Hunters International, an experience that she also wrote about. Joanna Gaines, the HGTV-famous decorator, got her start in an interview on the blog in 2012, long before “Fixer Upper” was a shiplapped twinkle in her eye. Before the pandemic, Alt Summit attracted thousands of crafty bloggers for multiday events at $1,000 per ticket. Ms. Blair’s first best-selling book, “Design Mom: How to Live With Kids: A Room-By-Room Guide,” was published in 2015.

The site still brings in about 150,000 unique visitors a month, a fraction of the millions she reaches on Twitter and Instagram, where she is chronicling the restoration of a 17th-century house she and her husband bought in 2019 in Argentan, France. “I’ve been known for lots of different things,” she said.

Ms. Blair acknowledged the gulf between her two books. The first book was “telling you, ‘here’s how to live with kids,’” and the new book is “‘here’s how to avoid having them,’” she said.

Her big pivot started in 2018 with a 63-tweet Twitter thread, the basis of “Ejaculate Responsibly.” Ms. Blair introduced herself in the first tweet, using her background as a Mormon mother as a selling point for the deliberately simple thesis that followed: Men need to stop putting the burden of birth control on women and stop getting them pregnant when they don’t want to be. Doing so will eliminate the need for abortions.

“I’m trying to shift the conversation, shift the abortion conversations to prevention,” she said.

Feminists have made the argument before and will make it again. Yet, the full thread, a thoroughly researched recitation of facts, like the number of days a month women are fertile (about two) vs. when men are (every day), went mega-viral, with nearly 21,000 retweets and more than 315,000 likes on the first tweet to date. Ms. Blair has gotten replies filled with vitriol and praise every day in the four years since.

“I’m much more likely to get people that don’t agree with me politically to listen to me if they think we’re on the same ‘team,’” Ms. Blair said. Though she is openly in favor of abortion rights and considers herself to have become more liberal since starting her blog, Ms. Blair deliberately played on assumptions that a Mormon mother with a large family would lean conservative. “Maybe I’m supposed to feel bad about it, but I don’t at all,” she said.

The Trajectory of a Mom Blogger

Before blogging, Ms. Blair majored in graphic design at Brigham Young University and was an art director at ad agencies in New York City. After the birth of her fifth child in 2006, and while experiencing a repeat bout of postpartum depression, she started Design Mom to find “community and validation” with other moms and “cute shoes that don’t fall off and don’t have Buzz Lightyear on them,” she said.

Kathryn Jezer-Morton, a Ph.D. candidate in sociology at Concordia University who is writing her dissertation on online motherhood (that’s to say, mommy blogs), described Ms. Blair as “one of the elder statesmothers.”

Ms. Blair’s readers were there when she had her sixth child, Flora June, who is now 12; when she moved from New York to France to Oakland then back to France; when her oldest, Ralph, now 25, left on his two-year Mormon mission trip to Colombia and when he came back; when she tried different ways to cope with depression; signed a contract to write a parenting book with her husband, Ben Blair, and then postponed it to write “Ejaculate Responsibly.” Ms. Blair blogged about it all.

As part of that original crop of “momfluencers,” Ms. Blair is paving the way for her peers, showing how your content can change as your life does, Ms. Jezer-Morton said. “She’s showing us a little bit about how this trajectory can last into later middle age, where you’re not parenting babies anymore, and you’re kind of moving on with your life.”

After the first viral thread, Ms. Blair posted about more political topics on Twitter, including gun safety, child welfare and Donald Trump; she changed her bio there to include “Thoughtful threads” in addition to “Life in France” and “Mother of 6.”

Recent posts on her Instagram feed, where she has 167,000 followers, reflect duality: One video shows some of her children making hand-shaped cookies with their grandmother. A subsequent photo captures a daughter protesting in Paris, wearing a shirt that says “USA Hates Women” and holding a sign that reads “Hold men accountable,” with a doodle of a fist yanking a sperm back by the tail. On her blog, Ms. Blair tackles subjects like hot flashes and panic attacks while also posting house tours and festive desserts for two.

“Yes, there’s been backlash,” Ms. Blair said. Since she posted the ejaculation thread, she said people tweet at her or try to argue with her, her children or siblings in public. She has a public-facing email address, and trolls use it as often as regular readers. The book’s publication has also opened her up to criticism. The Washington Post called the book “incisive,” but said, “The analogies can sometimes be a little silly, losing their power to illustrate a valid point.” “Most of the backlash is easy for me to handle,” she said, pointing out that an ocean now serves as a literal buffer.

Ms. Blair is certainly not the only blogger to have moved from kid stuff to life stuff. Ms. Jezer-Morton likened her to Glennon Doyle, the best-selling author who started her own Mormon mom blog a few years after Ms. Blair and who became known for being authentic and relatable. Ms. Doyle has shared her journey of divorcing her husband, falling in love with and then marrying Abby Wambach, the former U.S. women’s national soccer team standout.

The social media landscape has also shifted away from blogging and more toward shorter-form platforms, like Instagram. The days of the ultracurated, high-gloss veneers of perfect lives and ever-smiling, tidy children on mommy blogs, and across social media more broadly, are numbered, Ms. Jezer-Morton said. “Now, you’re expected to talk about things.”

Ms. Blair noted that her blog’s tag line was and has always been “the intersection of design and motherhood”; she described design as “problem-solving,” more than just picking the right size of area rug or accent wall color. “I quickly discovered I can write about anything under those two categories,” she said. “So while I did have a lot of how-tos and recipes and advice, parenting advice, and all that kind of stuff, I was also talking about what was happening in the world.”

The mantra that the personal is political and vice versa holds true for her; Ms. Blair readily shares whether she has had an abortion (she hasn’t), the many methods of birth control she has used over the years and how many times she has been pregnant (six).

“I don’t feel shame around any of those topics,” she said. The secrecy around abortion and contraception is another part of the burden she believes women have carried.

Still, she had never planned to tackle abortion so publicly.

A Twitter Thread and 28 Arguments Later

Months before she posted the thread that blew up, she had stashed it in her drafts for months. The start of the confirmation hearings that would ultimately place Justice Brett M. Kavanaugh on the Supreme Court was when Ms. Blair hit publish.

“I was so angry and just irritated at seeing man after man — you know, typically, male politicians — grandstanding about abortion,” she said.

The first tweet reads:

And dozens of tweets later, detailing the ways women are unfairly expected to bear the full burden of something that by definition takes two, she ends with:

Among the immediate responses were agents and editors wondering if she’d write a book encapsulating her arguments, an idea Ms. Blair said she dismissed outright. “Like, I said what I said. I don’t really see a book.” She also wasn’t sure she wanted to link her name forever to the word “abortion” in the Library of Congress.

“I’ve had my work get out in the world for all sorts of reasons during the pandemic. People were watching my house renovation religiously like it was a TV show; I had 40,000 viewers a day,” she said of posting her renovation on Instagram Stories. “I was hesitant, like, ‘Is this the thing that I’m going to be the most known for?’”

That her main hesitation was related to reputation, and not safety, is telling. “It’s one of those things where it doesn’t require any kind of courage or bravery from me, where it might from someone else,” she said. “Me getting out and answering these questions or speaking frankly about these topics — it doesn’t risk my marriage, it doesn’t risk my relationship with my children. It doesn’t risk my family relationships in any way.”

She can work from anywhere in the world and has owned homes in the Bay Area, France and New York. She is also white. Black women face a higher rate of maternal mortality than other groups do and account for nearly 40 percent of abortion patients. Ms. Blair has less at stake, whether she’s speaking about abortion or if she needed to seek one.

And so, her hesitation dissolved when the draft Supreme Court opinion overturning Roe vs. Wade leaked. “Yeah, let’s sell the book,” she remembered thinking. Ms. Blair wanted to make a tangible version of her argument.

She is now focused on getting the book into people’s hands. It contains 28 “arguments,” such as “No. 7: Society clings to the idea that men hate condoms.” Each argument starts with an all-caps, Instagram-ready red-and-black declaration in Avant Garde font, just waiting to be captioned: “This!” (Or, to be recreated, like embroidery on a vintage handkerchief.)

Ms. Blair has shared posts from Instagram followers saying they’re getting copies of the book for the men in their lives and for local high schools. There is also “Ejaculate Responsibly” merch, including an “I <3 Vasectomies” bumper sticker. (A portion of the proceeds goes to the Condom Collective, an organization that distributes a half-million condoms to college students each year.) On the blog, Ms. Blair has provided short form letters for people to send, with the book, directly to policymakers. One version of the letter even adheres to Amazon’s 213-character gift message limit.

According to a voluntary Google form that people can fill out on her site to help her keep track, Justices Amy Coney Barrett and Kavanaugh have been sent at least 20 copies each. Ms. Blair said that she had not yet heard from any politicians or justices about the book. “I was hoping,” she said.

And though she has her detractors and trolls, Ms. Blair said that the most negative responses so far to the book were less angry and more begrudging: “I’ve been getting emails from people who say, ‘I do not agree with you politically, but there’s nothing in this book that I disagree with.’”

Or, as she said with a laugh from the stage at Alt Summit, “It’s also a hard title to argue with. What are you gonna say, ‘I think people should have the right to ejaculate irresponsibly’?”

Kase Wickman is a culture and entertainment journalist, and the author of “Bring It On: The Complete Story of the Cheerleading Movie That Changed, Like, Everything (No, Seriously).”