The model and cookbook author Chrissy Teigen said late Wednesday that she and her husband, the singer John Legend, had lost their third child, a son, days after she posted on social media about suffering pregnancy complications.

“Driving home from the hospital with no baby,” Ms. Teigen wrote on Twitter. “How can this be real.”

Ms. Teigen also posted a black-and-white photograph on Twitter and Instagram that showed her sitting on a hospital bed with her hands clasped in a prayer-like position, seemingly in tears. Another Instagram photo shows her holding her baby as Mr. Legend kisses her shoulder.

Ms. Teigen, 34, joined a long list of celebrities who have broken a social taboo in recent years to speak out about pregnancy loss. Others include the former first lady Michelle Obama, the singers Beyoncé and Celine Dion, the actresses Brooke Shields and Kirstie Alley and the actors Hugh Jackman and James Van Der Beek.

Such disclosures have resonated with women across the United States, where pregnancy discrimination is widespread, and organizations that provide family-planning or abortion services are often targeted by conservative officials, and miscarriages are still largely spoken of in hushed tones.

“What nobody tells you is that miscarriage happens all the time, to more women than you’d ever guess, given the relative silence around it,” Mrs. Obama wrote in “Becoming,” her 2018 memoir. “I learned this only after I mentioned that I’d miscarried to a couple of friends, who responded by heaping me with love and support and also their own miscarriage stories.”

It was not clear how far along Ms. Teigen had been in her pregnancy. Last month, she let it slip that she was expecting a baby boy.

This week, she said on Instagram that she was hospitalized on Sunday because of excessive bleeding from her placenta. She also posted a picture of a blood transfusion pamphlet.

In an emotional post, Ms. Teigen wrote on Wednesday that she and Mr. Legend, 41, had not decided on their other two children’s names until just before they left the hospital. The couple have one daughter, Luna, 4, and a son, Miles, 2.

“But we, for some reason, had started to call this little guy in my belly Jack,” she told her combined 44 million followers on the platforms. “So he will always be Jack to us.”

In many ways, Ms. Teigen and Mr. Legend, who live in Los Angeles and own property in New York City, are the quintessential American celebrity power couple for the social media age.

She began carving out a public profile a decade ago, notably as a swimsuit model for Sports Illustrated. Her profile grew beyond the fashion world, and she used her love of cooking to publish cookbooks.

Ms. Teigen amassed millions of followers who praised her candid statements on the platform, her irreverent sense of humor and her willingness not to back down from a fight.