The policy will only allow athletes who've transitioned before the age of 12.
A stance on transgender athletes made by one of the world's most important athletic federations has sent ripples throughout the rest of the sports world and some fear it may bring even wider changes to come.
The governing body of international competitive swimming announced a policy that will only allow athletes who've transitioned before the age of 12 to take part in any of the elite international swimming competitions.
Lia Thomas, second left, of Penn University and transgender swimmer Iszac Henig (L) of Yale pose with their medals after placing first and second in the 100-yard freestyle swimming race at the 2022 Ivy League Women's Swimming & Diving Championships at Harvard University in Cambridge, Massachusetts.
Joseph Prezioso/AFP via Getty Images, FILE
The decision made by FINA pointed to what the organization says is a "performance gap" that emerges between biological males and females during puberty.
"Without eligibility standards based on biological sex or sex-linked traits, we are very unlikely to see biological females in finals, on podiums, or in championship positions," read the statement in part.
The policy also includes a proposal for a new open competition category, which athletes "would be able to compete without regard to their sex, their legal gender or their gender identity."
FINA cited coming to the decision after consulting with scientists and policy makers, but the policy still sent shockwaves throughout the world of swimming and beyond.
USA Wrestling and the International Rugby League have already followed suit and announced similar policies and other governing bodies are likely to follow suit.
FIFA, soccer's governing body, and World Athletics, the international governing body that covers track and field events, also announced a review of their transgender athlete policy.
Transgender athlete Schuyler Bailar said the FINA policy is "extreme."
"It's the most extreme policy that I've read to date.
I think it builds upon the discrimination that we're seeing specifically on trans people," said Bailar, who was the first openly transgender swimmer in NCAA Division 1 competition and the first trans man to compete in any NCAA Division 1 sport, to ABC News.
LGBTQ rights supporters gather at the Texas State Capitol to protest state Republican-led efforts to pass legislation that would restrict the participation of transgender student athletes on the first day of the 87th Legislature's third special session on September 20, 2021 in Austin, Texas.
Tamir Kalifa/Getty Images, FILE
The decision could impact athletes like Lia Thomas, whose record-breaking season in the women's swim category set off a firestorm of international controversy.
Thomas, who was recruited to the University of Pennsylvania men's swim team and competed with them for three seasons, began transitioning in 2019 and joined the women's team for the 2021 to 2022 season.
Earlier this year, Thomas made history as the first transgender athlete to win a NCAA Division 1 National championship.
That season, she set Ivy league records and rose significantly in the women's rankings versus her performance in the men's field.
In her only television interview, Thomas spoke to ABC News in May.
"Trans women competing in women's sports does not threaten women's sports as a whole," said Thomas.
"Trans women are a very small minority of all athletes and we haven't seen any massive wave of trans women dominating."
At the time, Thomas told ABC News that her goal was to swim at the Olympic Trials.
But now, the new policy change prohibits Thomas from achieving those dreams.
In a statement to ABC News, Thomas said, "The new FINA release is deeply upsetting.
It is discriminatory and will only serve to harm all women."
University of Pennsylvania athlete Lia Thomas prepares for the 500 meter freestyle at the NCAA Swimming and Diving Championships, March 17, 2022, at Georgia Tech in Atlanta.
John Bazemore/AP, FILE
Former Southern Illinois University swimmer Natalie Fahey was one of the first openly trans women to compete in the NCAA.
Unlike Thomas, Fahey didn't break any records and her races were far less controversial.
"After I transitioned, I was solidly middle ground.
I didn't come in and break any records," said Fahey.
"I only competed at a conference, but certainly just the fact that I'm not as good as Lia is, weighs into that."
Fahey added that being able to swim as a woman was crucial to her.
Three-time Olympic Gold-medalist Nancy Hogshead-Makar is the founder of Champion Women, a non-profit organization that advocates for gender equality in sports for women and girls.
She said she advocates for the open competition category.
"Trans women are women, but there are a few places where biology really matters and women's sports is one of them," she said.
But Bailar argues that the policy will have a lasting impact on trans youth that expands beyond sports.
"Most people play sports for fun, to learn cooperation, to have a team, to have peer mentorship," said Bailar.
"This [policy] polices all women because you have to know which girls are trans in order to exclude them.
And when you do that, you enforce the policing of all women's bodies."
