Around 30 per cent of twins are incorrectly classified as either identical or fraternal, new research shows.
Now many parents are using DNA tests to double check.
When Sue Sukkel gave birth to her daughters, Abbie and Lilly, doctors said they were fraternal or genetically distinct, compared to identical twins.
Ms Sukkel said it was difficult explaining to strangers why her daughters looked so similar.
"When they were born, I constantly got stopped in the street, constant questions. Are they identical?'," she said.
But as the girls grew, so did their similarities.
"It's like being the same. And plus you can know what you look like. You can just have a mirror walking with you every second of the day," Abbie Sukkel said.
"When they were born, I constantly got stopped in the street, constant questions."
Three years ago DNA tests revealed their status as identical.
"When I got the letter saying they were identical, it was so overwhelming that I burst into tears, because all of a sudden everything made sense," Ms Sukkel said.
Doctor Jeff Craig of the University of Melbourne's Australian Twin Registry said there was a lot of misconception surrounding the genetic similarities of twins.
"Some people believe that to be identical you have to look identical and behave identical, and if you're not then you must be fraternal, that's wrong. You can get identical twins that look and behave quite differently," he said.
"The second assumption starts in the womb, when there's two placentas. You cannot say those are identical or fraternal, they can be either."
Doctor Craig said 29 per cent of fraternal twins were led to believe they were identical or were unsure.
Elizabeth and Shaun Stow's boys Hamish and Fraser, who were thought to be identical, also suffered health problems that mostly affect identical twins.
"When I got the letter saying they were identical, it was so overwhelming that I burst into tears, because all of a sudden everything made sense."
"The boys developed twin-to-twin transfusion syndrome, which was something that happened with identical twins," she said. "Fraser took all the nutrients and left Hamish with none."
But as the boys grew, their looks became different, and two years ago DNA tests revealed they were fraternal.
"It was just nice I could tell people the truth. That I could say, 'yes they're fraternal twins'," said Ms Stow.
Researchers said it was important to know the genetic status of twins for mental and physical health reasons.
University of Melbourne's Australian Twin Registry's Doctor Jeff Craig said identical twins could share a range of medical issues including autism.
"If one of [the twins] develops a disease and they're identical, the other one should be screened for that disease as they're more likely to have it. In some cases it can be a life or death situation," he said.
But Doctor Craig added that having a genetic match in the family had positives.
"If one of them needs a transplant, the other is probably a perfect match. One of our twins said, 'Well now I know where I can go if I need a spare kidney'."
Genetic testing of same-sex twins is recommended as early as possible.
