Rumors have been swirling that Princess Diana had a daughter before she gave birth to her eldest child, Prince William. Only, she never met this girl, who was named Sarah, because she died before finding out about her.

A royal fan named Charlotte Dobbs claimed in a post on Quora that Prince William and Duchess Kate have met Sarah in New York. She wasn't sure, however, if Prince Harry has met his alleged sister as she has been elusive and hiding from her real identity from the public. 

But, how did this rumor about Princess Diana's secret daughter started? It all goes back to before Princess Diana and Prince Charles were set to be married. 

Allegedly, she had her eggs harvested and frozen during a gynecological test ordered by Queen Elizabeth before her marriage to Prince Charles. According to Daily Mail, the test was necessary to determine fertility.  

By her own admission, Princess Diana, allegedly, told a close friend that she was asked to undergo a fertility test before the wedding. Once it was a proven success, Princess Diana's engagement to Prince Charles was announced.

The embryos, on the other hand, were ordered to be destroyed. However, the doctor supposedly went "rogue" used Princess Diana's eggs for his own wife. That egg became a baby girl, allegedly born in 1981 or some eight months before Prince William's birth. Her regular parents gave her the name Sarah, and she grew up to look so similar to her supposed biological mother.

When Sarah was in her early 20s, she lost her parents in a car accident. She, eventually, learned the truth about her birth and sought to trace her real parents. However, Sarah, allegedly, received death threats while looking into her past. So scared was she that Sarah moved to America and changed her appearance.

The Daily Mail report also stated that the origin of the story is actually based on a fiction novel penned by Nancy E. Ryan. The author was inspired to write "The Disappearance Of Olivia" in 2011 because of her fascination with Louise Joy Brown, the first baby born via IVF in 1978

Ryan also said that she leaned towards Princess Diana because the late royal used to talk about wanting to have a daughter in many interviews. Being one of the world's most fascinating women also made Diana a character in her book. 

Fans of the People's Princess has lapped up Ryan's novel as the truth, even if it was clearly a work of fiction. Eventually, the tabloid press picked up on it and also wrote stories about Sarah as fact despite the absence of proof.