tonight -- >> being trans is not a choice. i didn't have any other choice. >> leah thomas, the u-penn swimmer at the center of a national firestorm over trans women in sports speaks exclusively with us at abc news and espn in her first on-camera interview. >> trans women very much can compete. it's not a threat to women's sports. >> but some like tennis legend martina navratilova disagree. >> it's just not a fair fight. it's that simple. >> now after leah's history-making season, her supporters speak out. >> people are throwing a fit because leah's good. >> weighing into the controversy. >> they're putting ideology

ahead of opportunity for women athletes. >> what would you say if you could in response? >> this "nightline" special edition, "leah thomas: the interview," will be right back. on travel purchased through chase with chase freedom unlimited. i earn 5% on our cabin. hello cashback! hello, kevin hart! earn big time with chase freedom unlimited with no annual fee. how do you cashback? chase. make more of what's yours.

♪ when i dive into the pool, nothing else matters.

it's a moment of clarity. it's almost like you're flying. >> and we're off in the final of the 200-yard freestyle -- >> it's been somewhat surreal to see my name pop up a lot. >> earlier this morning, leah thomas had the fastest time -- >> i try to ignore a lot of the noise. people will say, oh, she just transitioned so she could win. trans people don't transition for athletics. >> she's in good shape with a -- >> we just want to be able to live our lives. >> reporter: for years, this room has served as a sanctuary for leah thomas. she's logged hundreds, even thousands of hours here. some good, some not. >> all i was thinking was that i can't live like this anymore. >> you thought about quitting? >> yeah, it was a very tough time. >> reporter: until now, she's kept her head down, quietly making history. the first openly transgender woman to become a division i

ncaa champion. >> leah thomas will capture the win -- >> igniting a fierce national debate about fairness in women's sports. transgender rights and identity. >> being trans is not a choice. i didn't have any other choice. >> leah thomas grew up as a biological boy -- >> sparking widespread outrage -- >> reporter: leah thomas, one of the most controversial athletes in the country, is ready to talk. what makes you want to break your silence now? to respond to critics. >> stop this woken sanity now. >> reporter: to take on the question at the heart of it all, is it fair to let trans women compete? >> trans women very much can compete. it's not a threat to women's sports. >> the male biology, that advantage is built in, it's baked in. >> if this isn't stopped, this is the end to women's sports at a competitive level. >> do you think it should come down to a choice between your

sport or your gender identity? >> no, i don't want other trans people to face that, that choice. >> what's it like saying your name? >> now that i am leah now, it's really meaningful. it feels very authentic. >> in the midst of all of this spotlight on you, i can't imagine this is your comfort zone. >> it's not exactly. during the season i would just ignore it. i wouldn't really read any of the articles that came out. i certainly wouldn't read any comments or things people had to say. >> reporter: growing up, leah says she was a happy child but always felt there was a disconnect. >> i didn't feel like i was a boy. i was like, this isn't me, this isn't who i am. >> how old were you when you first had that inkling?

>> i think 7 or 8. and then again in middle school. >> reporter: it wasn't until she arrived at the university of pennsylvania that those feelings became impossible to ignore. >> freshman year of college is when i delved into reading the experiences of other trans women. at first there's immense relief where you're like, everything sort of makes sense. and then you start to think about having to come out to people, how they might react. i couldn't ignore it anymore. and i had to finally and fully except for myself that i am trans. >> reporter: by sophomore year, leah achieved her best rankings yet in the men's field. but mentally, things were worse than ever. >> i was extremely depressed. i was barely going to classes. i could really barely get out of bed. i said, i can't live like this anymore. >> you thought about quitting swimming altogether? >> yeah, it was a very tough time. >> why? >> i was scared of how my

coaches and my teammates would react. >> i'm sure there are plenty of people who don't understand what you were going through, who might say, why didn't you just stick it out one more year in the men's team? >> i wouldn't have been able to. i was very, very miserable. i struggled with suicidal thoughts. being trans is not a choice. i didn't have any other choice. >> reporter: by may of sophomore year, 2019, leah started the medical part of her transition, hormone replacement therapy. >> when i started hrt, i had accepted that i probably wouldn't swim again and that my career was over. and then that summer i was feeling a lot better mentally and emotionally. i realized just how much this sport really means to me. and i didn't want to give it up. >> reporter: that required facing her fears and coming out to her team. what did you say to them?

>> basically, like, i'm transgender. i'm not a man, i'm a woman. once i hit the ncaa requirement, i'll be switching teams. >> what was the reaction? >> in that moment it was a lot of support. >> how quickly physiologically did things change for you? >> slowly at first. the quickest thing that happened is i lost muscle mass and i became a lot weaker and a lot slower in the water. >> reporter: ncaa rules for transgender women had been in place since 2010 and required athletes to be on hormone therapy, including testosterone suppression, for at least one year. by the time leah started competing on the women's team, she'd been on hormone therapy for more than two years. >> i knew there would be scrutiny against me if i competed as a woman. >> did you expect this much scrutiny? >> i did not expect it would reach quite the level that it did. >> we have a problem with trans women competing as women because they have a baked-in advantage.

it's just really biological reality. you cannot deny that. >> reporter: martina navratilova knows what it takes to be an elite athlete. with 18 grand slam singles titles, she's widely considered one of the greatest tennis players of all-time. the tennis legend is part of the women's sports policy working group, an organization that aims to influence rules on women's sports, specifically how or if to include trans women. what opportunities do you think leah thomas' victories take away from cisgendered women? >> the ones that would have been on the podium if leah was not racing. the ones that would have gone to the national championships if leah was not on the team. you don't have to dominate, you have to win to take away scholarships, et cetera. >> reporter: the battle over fairness in women's sports has intensified over the last four years. so far 18 states have banned or limited transgender kids' participation in school sports. leah found herself in the center of the fight, especially after she started winning.

she rose significantly in the women's rankings. in some cases number one nationally. >> there are some who look at the data and suggest your enjoying a competitive advantage. >> first of all, there's a lot of factors that go into a race. and the biggest change for me is that i'm happy. and it's also been three years since i last fully competed. and three years of training can also lead to some improvement. >> some of the critics would say, oh, you transitioned in order to be successful. >> trans people don't transition for athletics. that's not something i ever considered when i was transitioning. >> reporter: at a meet in december, leah posted the nation's fastest times in the 200 and 500 freestyle. her success in the water was met with signifcant pushback. 16 of her teammates and some of their parents wrote anonymous letters to the ncaa and the ivy

league arguing leah posed a threat to women's sports. >> trans women competing in women's sports does not threaten woman's sports as a whole. >> why? >> because trans women are a very small minority of all athletes, and we haven't seen any massive wave of trans women dominating. >> abc news spoke with one penn swimmer and her parents who said they don't blame leah, they just want to protect opportunities for other women. >> the letter anonymously said that they absolutely supported your right to transition, but they simply think it's unfair for you to compete against cisgendered women. >> you can't go halfway and be like, i support trans women and trans people but only to a certain point, only to a certain extent. >> reporter: then in january, a new roadblock. the ncaa updating its policy on transgender athletes, putting leah's season in jeopardy. >> this morning the ncaa facing criticism after adopting new

rules for transgender athletes. >> reporter: but eventually decided not to impose the new rules mid-season, clearing the way for leah to go to nationals. >> she was pushed -- >> reporter: she won the 500 freestyle. >> the ncaa championship! >> reporter: making history as the first transgender athlete to win a division i national championship. what was that moment like for you? >> it was an incredible experience. >> reporter: immediately afterwards, second and third place finishers, emma wyant and erika sullivan, congratulated leah. but in a show over how political the fight over trans athletes has come, former vice president mike wens and florida governor ron desantis called emma weyant the true winner. >> the ncaa is taking efforts to destroy women's athletics. >> what would you say in response? >> i don't really have anything to say to them.

they're transphobic and they're not experts on trans people and trans bodies. >> they don't give a damn about women athletes. they just pretend to care. it's totally political. >> reporter: leah went on to take eighth place in the 100 freestyle and tied for fifth in the 200. >> it's a wire-to-wire win for taylor! >> reporter: the woman who shared that information, riley gains. >> i didn't think whenever i first heard of her that she would swim in the ncaas, i just thought they'll handle this. >> reporter: though they tied, leah was given the fifth place trophy, riley's was mailed to her. the ncaa has rules for ties and leah's name was listed first on the score sheet. riley says an official told her they were doing the results in chronological order. >> what do you mean, chronologcal order? we just -- we tied. so at this point i realized, you know, they're doing this to save face. so not only were women kind of put in this position where we have biological males competing

against us, we were put on the back burner for biological males. >> reporter: riley, an all-american, says it wasn't about the trophy but how the ncaa handled the situation. >> i don't think that leah necessarily did anything wrong, you know, i think she was following the rules that were set for her. >> what would you say to folks who say, see, if you stop swimming, people wouldn't be so angry? >> i'm a swimmer at heart. it's what i love to do. it's a part of who i am. i don't want to give that up. up next, is leah thomas' swimming career over? are the olympics in your future? plus the debate over whether trans women have a physical advantage in the pool. >> biology rules in sports. my a1c stayed here, it needed to be here. ruby's a1c is down with rybelsus®. my a1c wasn't at goal, now i'm down with rybelsus®. mom's a1c is down with rybelsus®.

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prevagen. healthier brain. better life. "nightline," "lia thomas: the interview" continues. here again, juju chang. >> i will watch myself at times staring in the mirror. because you spend 20 years avoiding any mirror. when you finally feel at peace and connected with your body, it's incredibly liberating. >> at 23, lia thomas says she's finally at peace with herself. but the sudden onslaught of attention has left her depending on friends like carol baylor. he was the first openly transgender swimmer in ncaa division competition and the first trans man to compete in any ncaa sport. >> leah's not the first openly trans woman to compete in swimming. people are throwing a fit because leah is good.

>> reporter: leah's success sparking heated debate about women's sports. critics say this is about protecting opportunities for girls and women. >> trans women argue they are women and that their rights need to be protected as well. what would you say to that? >> i think in real life, absolutely, 100% equality, 100% inclusion in every way. but sports is divided by categories. and the biggest category is the sex category. biology rules in sports. so yes, you can be included and competing, but you need to do it against biological males. it's just not a fair fight, it's that simple. >> if you're wanting to protect women, you need to let trans women play. trans women are women, so we need to include everybody. >> reporter: though she has broken ivy league records there was worry lia would break ncaa national records. >> have you done that so far? >> no, i haven't broken any national records. some people have this sort of sensationalized and stereotyped

view of trans women. and that's just not accurate. >> reporter: in fact, lia says her body and her athletic advantages have been negatively impacted by hormone replacement therapy. >> one of the biggest was loss of muscle mass. i had a lot of fat redistribution. i shrunk, actually, probably about an inch in height. my feet got smaller. so it really does change your entire body. >> reporter: but some in the medical field say those changes may not be enough. what are the effects of tells is to treason on the male body? >> the first thing i think that's really important what is it does to skeletal muscle. >> reporter: dr. michael joiner is a professor at mayo clinic, a physiologist who studies the differences between is sexes. >> it makes your muscles bigger, stronger, more responsive to training. second, it can increase the red blood cells in your body which permit you to transport more

oxygen. it probably increases the size of your lungs. and then it also does things like make your heart larger. >> reporter: joiner says the legacy of effect of testosterone, which is higher in males than females, can be lasting. what are the physical aspects that trans women may not be able to roll back with hormone therapy? >> obviously, issues related to body size, airway size, hand size, foot size, perhaps bone density, and so forth. but i think the main thing is just the interactions of exercise training and skeletal muscle. >> are you saying that years of hormone therapy cannot put trans women in a place to compete with cisgendered women? >> i think the evidence so far would suggest that a period of a year, two years, three years, four years, is probably insufficient. >> most important thing i would say to any sports governing body is the science is in. >> reporter: joanna harper has

advised some of the leading sports bodies on policy por trans athletes, including the international olympic company. >> understand that any policy you create should be considered a living document. >> reporter: she's currently studying trans athletes and agrees while trans women do retain some advantages from male puberty, hormone therapy can make it possible for them to compete with cisgender women. >> any advantages that trans women have over ciswomen after hormone therapy are small enough that it's not the same thing as having women facing men. >> reporter: while the science on trans athletes is evolving, the debate continues raging. >> i choose to be public to help other trans people and to be able to show trans people that you don't have to pick between being your authentic self and doing what you love, that you can do both. >> i think lia's success is

already saving lives, to put it as bluntly as possible. i've had messages in my inbook saying, can you tell lia my child feels safer because lia is in the world? they know they can be themselves because lia is herself. >> how do you see your future? >> going to law school. fighting for trans rights and trans equality is something that i've become much more passionate about and want to pursue. >> are there olympics in your future? >> i intend to keep swimming. it's been a goal of mine to swim at olympic trials for a very long time. i would love to see that through. put. assumed dust always stad turns out? it can be on the move. we were breathing that day and night! that's when we started using swiffer. in just a few minutes, duster captures dust before it gets airborne. it traps and locks dust in one swipe.

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