0131_beyonce_singleBefore giving birth with daughter Blue Ivy, Beyonce Knowles had a miscarriage ... and in her new HBO documentary, she opens up about the traumatic experience for the first time.

"About two years ago, I was pregnant for the first time," the singer reportedly says in "Life Is But a Dream." "And I heard the heartbeat, which was the most beautiful music I ever heard in my life."

"I picked out names," Beyonce adds. "I envisioned what my child would look like ... I was feeling very maternal."

But then, the unexpected happened.

"I flew back to New York to get my check up -- and no heartbeat," she explains. "Literally the week before I went to the doctor, everything was fine, but there was no heartbeat."

Jay-Z briefly referenced the miscarriage in a song called "Glory" last year ... and Bey says she turned to music to express herself too.

"I went into the studio and wrote the saddest song I've ever written in my life," she says of the tune, which will be on her next album. "It was the best form of therapy for me, because it was the saddest thing I've ever been through."

Knowles' new album is due out this year (as soon as April) and she'll be performing this weekend at the Super Bowl.

Check out some rehearsal video for the big day below, which includes some "Single Ladies" and "Crazy In Love" dance moves.

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