Kim was 9 when she found out she'd been switched at birth with Arlena. Arlena had just died.

Feature image: The Times/ABC 20/20.

In 1978, two women gave birth at a hospital in Florida.

Barbara Mays gave birth to her first child, a baby girl with a severe heart condition. Three days later, Regina Twigg also gave birth to a healthy baby girl – she already had a big family.

The babies were switched and given to the opposite mothers to take home. A family practitioner at the hospital ordered a nurse’s aide to switch their ID bands, because they felt sorry for Barbara who’d been trying for a baby for years and had given birth to a child who wasn’t likely to live long.

Here’s a trailer for the two hour special delving into the story. Post continues after video.

Kim and Arlena grew up with their respective families – Kim an only child, and Arlena one of seven. It was only when the girls were nine – and Arlena died from heart failure – that the mix-up was discovered.

The family learned that Arlena might not be their biological child, when tests taken before her surgery showed her having a different blood type to both her parents.

After her death, Regina and Ernest Twigg started searching for their biological daughter.

It was Bob, Kim's father, who broke the news to his then nearly 10-year-old after the couple found him. Barbara had died from cancer when Kim was only two.

In a new interview on ABC's 20/20, Kim explained: "He had to sit me down and was like, ‘Look, there's another family. Their little girl died. Her blood did not match theirs. You were the only other baby born at Hardee Memorial Hospital, and they found you through a detective".

“[He said] ‘I just want to let you know that you are my daughter. I love you, no matter what the tests come back, but you have to take a blood test,’" she added.

It was also a huge shock for the Twiggs. "I was very shocked,” Irisa Roylance, the oldest of the Twigg kids told the ABC. "But she was my sister, so it didn’t matter to me."

In previews for the ABC special, all of the Twigg children relive memories of their sister and the grief that lives with them to this day from her passing.

Kim opened up about what that time was like for her - a tumultuous youth and a new identity that was not just thrust upon her, but splashed across the media of the day.

Cries of "I want my life back" and "my life is a Jerry Springer show" made headlines across the country.

The now 40-year-old Kim told the ABC she has tried to “move forward the best (she) can”. She said she tries not to dwell on the past and have anger.

The story of the switch first made headlines in 1988 when the families sued the hospital. Eventually they were awarded multi-million dollar settlements.

Regina also pushed for the right to get to know her biological daughter. But after the fifth visit, Bob cancelled the next meeting. He said Kim's grades were suffering and it was having too much of an emotional impact on her.

Bob's third wife Darlene told the program, "Kim might not have been Bob's biological daughter, but in every other sense of the word she was his daughter and protecting and keeping her close, you don't just walk away from that".

In 1993, when Kim was 14, she petitioned the state judge for a legal "divorce" from her biological parents with the intent to deny them visitation rights. She won.

Three decades on, Kim recognises her mistake. "I wanted to know about the Twiggs. I wanted to know what my biological family was like. I regret divorcing the Twiggs."

It was also in 1993 when a nurse's aide - that was by this point on her deathbed - came forward to admit she'd been the one that'd switched the ID bracelets after being ordered to by a doctor. Regina Twigg wanted to pursue criminal charges but the statute of limitations had run out.

Kim grew up feeling like she didn't really have a mum at all. Until she was six, she thought her dad's second wife was her mum. Then he told her about Barbara. Then aged 10 she found out about Regina.

“I had a rough childhood,” she told the documentary.

When she was 15, Kim ran away from home and sought refuge at the Twiggs. She ended up staying a year and a half.

At 18, she sold her interest in the money she received from the hospital to an annuity company in a structured settlement. As a result she can't get access to it until she's 70.

It was around this time that she married her first husband, who she had a son with. They divorced and her child was raised by her ex-husband and his family.

“I was young. I didn't know how to mother. I didn't know how to take a crying child and I made mistakes in my life that I can't undo. But I knew he was going to be cared for and loved, and that's all I wanted for him," she told the ABC.

Fast forward to the present day and Kim is five kids from her second marriage and has plans to get her high school diploma.

Her relationships with the two remaining mother figures in her life, Regina (her biological mum) and Darlena (Bob's third wife) remain strained and she doesn't keep in touch very often. Bob died in 2012, but Kim hadn't been in contact with him for years before his death.

As for Arlena - the other little girl at the centre of this story - Kim often wonders about her.

"I feel bad for Arlena, because here it is, I'm 40. She would have been 40. I heard so many good stories about her, that she was really sweet… her life was way too short.”

The full story airs Friday (American time) on ABC.