Mandy Moore has opened up about her surprise pregnancy, saying she didn't think she could fall pregnant unassisted.
The This is Us star is expecting her first child, a boy, with husband Taylor Goldsmith in the next few weeks, but said she had a hard time believing she was pregnant at first.
The condition is caused by tissue - similar to the lining of the uterus - that grows outside of the uterus and can cause heavy and painful periods, as well as infertility.
Moore said she had been planning to have surgery to clear the tissue when she spontaneously fell pregnant.
"I was fully prepared to go have surgery and fix my uterus and hopefully get rid of the endometriosis, if it was there," she shared.
"It was nice to have a plan and to know, OK, well, this is why I haven't been pregnant yet."
As she had been told it would be difficult for her to fall pregnant without intervention, the actress said she had found it hard to believe at first.
"Because of this issue with my uterus, I was very hesitant to believe it and put any stock in it," she said.
"I sort of was holding my breath until 12 weeks."
The star announced the news in September on Instagram, posting a photo of Goldsmith holding her belly, saying: 'Baby Boy Goldsmith coming early 2021'.
Close to 700,000 women in Australia are believed to have endometriosis - though many others go undiagnosed.
Tissue can be found in areas such as the ovaries, fallopian tubes and peritoneum.
While some women experience no symptoms, for many it is a debilitating condition that can cause heavy bleeding, bleeding between periods and lethargy.
The condition can only be diagnosed through a laparoscopy (keyhole surgery), however diagnosis can take an average of seven years from symptoms developing.