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Beyonce has revealed one of her twins' hearts stopped, forcing her to undergo an emergency ceasarean delivery.

On Wednesday, the singer's documentary, Homecoming: A Film by Beyonce, dropped on Netflix, giving an inside look at her 2018 Coachella performance—the first black woman headliner in the festival's history—that featured dozens of dancers, and a marching band for a set that honoured the musical legacy and culture of historically black colleges (HBCUs) in the U.S.

In the film, the Formation star, who gave birth to twins Rumi and Sir Carter in June 2017, opens up personally and professionally about the legendary performance, and, according to The Hollywood Reporter, reveals she almost lost one of her babies.

"One of my babies' hearts stopped while in the womb, so I had to get an emergency C-section," she said. "There were days I thought I would never be the same."

Beyonce didn't give further details on which of the twins was affected. However, she has previously shared in an essay for U.S. Vogue that she and her youngest two spent weeks in hospital.

"I was 218 pounds the day I gave birth to Rumi and Sir," she wrote at the time. "I was swollen from toxemia and had been on bed rest for over a month. My health and my babies' health were in danger, so I had an emergency C-section. We spent many weeks in the NICU."

Following their births, the 37-year-old fought her way back to health, and the documentary details the work that she and her team put into her comeback performance the following year.

"What people don't see is the sacrifice. It was important that I brought our culture to Coachella. There was a four-month period of rehearsals with the band before we started the four months of dance rehearsals," she tells the camera in the film, which also features her family, including rare shots of the twins, now 22 months old, seven-year-old daughter Blue Ivy singing and intimate moments with husband JAY-Z.

"I feel we made something that made my daughter proud, made my mother proud, my father proud, and all of the people that are my brothers and sisters around the world and that's why I live," she says during her final voice over. "I'm so lucky and grateful that I'm able to take all these crazy ideas and actually make it into something that heals people and that may spark vision in people that shows them to dream big. Shows them that they are limitless. If my country a*s can do it, they can do it."

Beyonce also dropped album Homecoming, a previously unannounced live version of the concert, on Wednesday.