Princess Diana once witnessed her father 'slap' her mother during the royal's tumultuous childhood, CNN doc details

• During Princess Diana's unhappy childhood, her father once slapped her mother, a CNN doc detailed.
• "I remember seeing my father slap my mother across the face ... and she was crying," Diana said.
• This abuse led to the couple's later divorce, which would affect Diana until her untimely death.

Princess Diana's father once slapped her mother during an unhappy childhood, a new documentary series from CNN detailed.

"Diana," a six-part docuseries that premieres Sunday, tracks how the late British royal grew up in rural England in a household that would often turn violent.

The first episode, titled "The Girl from Norfolk," details how Diana was raised in the countryside.

Though Diana "adored being in the country" as she's heard saying in recordings for Andrew Morton's book, "Diana: In Her Own Words" that were featured in the documentary, she did not grow up in a happy home.

One point of contention was that Diana's mom, Frances Shand Kydd, and her husband, John Spencer, longed for a son to carry on the family legacy.

An aristocratic family like Diana's practiced primogeniture, which is when the firstborn son inherits the family estate, according to Julie Montagu, an American-born viscountess featured in the docuseries.

British journalist Bidisha Mamata added in the documentary that as a wife "you're supposed to keep popping out male children."

Princess Diana's parents tried very hard to birth a son, which left the royal feeling like a 'nuisance'

Diana's mother was successful in birthing two daughters, Sarah and Jane, before becoming pregnant with a boy. The family's firstborn son, John, lived only for hours before dying of a lung condition.

Eighteen months later, after trying for another son and grieving a child her husband so badly wanted, Kydd gave birth to Diana, the doc said.

Because she wasn't the hoped-for sex, Diana grew up feeling as if she had already disappointed her parents, according to Montagu in the docuseries.

"I couldn't understand why I was perhaps a nuisance to have around," Diana began in the doc with the royal later realizing it was because "the child before me died, and it was the son ... and heir."

"And then comes a third daughter," the princess continued. "'What a bore. We're going to have to try again.'"

Eventually however, Kydd and Spencer had their heir three years after Diana was born: Charles Spencer.

Diana's mother leaving her birthed a fear of losing her own children that 'played on Diana until her death,' Montagu said

After Kydd had "fulfilled her contract" as a wife to Spencer, "she began to plot an escape route for herself. Johnny could be violent, and she felt she and her children would be safer out of the home," Penny Junor, a journalist and author, said in the documentary.

"I remember seeing my father slap my mother across the face and I was hiding behind the door and she was crying," Diana recalled in the recording of her tumultuous childhood.

By 1967, Kydd told Spencer she was leaving and filed for divorce. She had full intention of taking her four children with her to remove them from their abusive father.

But Kydd's mother, Ruth Roche, was appalled that her daughter would leave Spencer, an attractive Earl and socialite. She testified against Kydd in the divorce proceedings, calling her a "bad mother," the docuseries detailed. In turn, Spencer got full custody of the four children and Kydd disappeared. (She'd later go onto marry again in 1969; this time to Peter Shand Kydd, the heir to a successful Australian wallpaper business.)

"Diana's mother, in a very unhappy marriage, left, for very good reasons, and was shunned in society," Montagu said in the docuseries.

Diana was incredibly affected by the loss of her mother especially since by age 6, her entire support system had vanished, experts featured in the documentary said.

But most of all, the fear of losing her own children plagued her, Montagu said.

"Because of what happened to Diana at such a young age and watching her own mother lose her children, this fear of losing children would have played on Diana until her death," she added.

CNN's six-part docuseries, "Diana," premieres on Sunday at 9 p.m. ET/PT.