Beyonce believes she was "blessed" with boy and girl twins to right the wrongs of her ancestor, a slave owner who married a slave.

The singer said she related the discovery about her family history to the arrival of her children, Rumi and Sir, who were born last year.

"I researched my ancestry recently and learned that I come from a slave owner who fell in love with and married a slave. I had to process that revelation over time. I now believe it's why God blessed me with twins. Male and female energy was able to coexist and grow in my blood for the first time."

Beyonce's maternal family are Creole, from Louisiana.

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The 36-year-old performer gave the most candid interview of her career to the September issue of US Vogue, which she guest edited. She discussed the "crazy" pressure to regain her figure following the birth of her first child, Blue Ivy, in 2012, which led to her going back on tour only three months later while still breastfeeding.

"After the twins, I approached things very differently.

"I was 218lb (99kg) the day I gave birth to Rumi and Sir. I was swollen from toxaemia 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," she said.

"My husband was a soldier and such a strong support system for me."

She chose Tyler Mitchell, 23, to photograph her for the September issue, making him the first African-American to shoot the cover of Vogue in its 125-year history.