Sixteen of transgender swimmer Lia Thomas' female teammates on the University of Pennsylvania team sent a letter to Penn and to the Ivy League, urging schools not to contest the new USA Swimming rules on competition for transgender athletes.

"We fully support Lia Thomas in her decision to affirm her gender identity and to transition from a man to a woman," the swimmers wrote. "Lia has every right to live her life authentically. However, we also recognize that when it comes to sports competition, that the biology of sex is a separate issue from someone’s gender identity." 

"Biologically, Lia holds an unfair advantage over competition in the women’s category, as evidenced by her rankings that have bounced from #462 as a male to #1 as a female," they added. "If she were to be eligible to compete against us, she could now break Penn, Ivy, and NCAA Women’s Swimming records; feats she could never have done as a male athlete."

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A parent of one of the 16, who spoke to Fox News Digital on the condition of anonymity, said that the girls were told not to speak to the media and that when they expressed concerns about the unfairness of competing against a large athlete like Thomas who had gone through male puberty, their concerns were dismissed.

Lia Thomas of the Pennsylvania Quakers gets ready to compete in a freestyle event during a tri-meet against the Yale Bulldogs and the Dartmouth Big Green at Sheerr Pool on the campus of the University of Pennsylvania on January 8, 2022 in Philadelphia, Pennsylvania. (Hunter Martin/Getty Images)

"It’s been incredibly frustrating, having to learn that our daughters were put in a situation where they were not allowed to speak about something that was so blatantly obvious to them every single day and find little support from the university other than ‘Hey, get some counseling,’" the parent told Fox News Digital. He said the university athletic department sent in a publications representative who essentially told the girls, "You should say nothing – here's your script: don't say a word."

"Our girls have worked their entire lives, a lot of them since they were 6 or 7, all with the hope of eventually getting to that higher level," the parent said. "Realistically, only a handful make it to the Olympics, but there is a really good opportunity for hundreds of girls to make it to the college level."

The parent said that competing against a male-to-female transgender athlete like Thomas represented an "extra burden." The parent called the experience of watching the girls have to compete with Thomas "maddening."

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The parent cast their struggle for fairness as a mental health issue.

"That’s the real bottom line: just because you are some sort of a ‘normal’ student athlete, doesn’t mean you don’t have a ton of pressure and stress and strain," the parent said. "It’s a very challenging school academically, and swimming, it’s very time intensive – they swim two workouts a day, oftentimes."

Lia Thomas of the Pennsylvania Quakers

Lia Thomas of the Pennsylvania Quakers smiles after winning the 200 yard freestyle event during a tri-meet against the Yale Bulldogs and the Dartmouth Big Green at Sheerr Pool on the campus of the University of Pennsylvania on January 8, 2022 in Philadelphia, Pennsylvania. (Hunter Martin/Getty Images)

"No one is unsympathetic toward Lia’s plight, but at the end of the day, it’s a competitive sport, and for the sake of fair competition, having Lia act as if she’s a female is just not fair," the parent said. The parent said the girls face a "pressure to conform" on the issue and that the team's "camaraderie has been severely disrupted."

The NCAA updated its transgender participation policy last month, saying it will be determined on a sport-by-sport basis. If there is no national governing body for the sport, then the NCAA sport will follow the International Olympic Committee’s (IOC) policy. On Tuesday, USA Swimming released new guidelines emphasizing fairness for cisgender females. 

The new guidelines aim to enable "transgender athlete participation in the U.S. that relies on science and medical evidence-based methods to provide a level-playing field for elite cisgender women, and to mitigate the advantages associated with male puberty and physiology."

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In order for a male-to-female transgender athlete to compete in an elite swimming event, the new policy requires a decision-making panel of three independent medical experts to determine eligibility based on two criteria: "Evidence that the prior physical development of the athlete as a male, as mitigated by any medical intervention, does not give the athlete a competitive advantage over the athlete’s cisgender female competitors"; and "Evidence that the concentration of testosterone in the athlete’s serum has been less than 5 nmol/L (as measured by liquid chromatography coupled with mass spectrometry) continuously for a period of at least thirty-six (36) months before the date of application."

"Athletes will need to abide by USA Swimming’s Athlete Inclusion, Competitive Equity and Eligibility Policy to be eligible to set USA Swimming National Age-Group Records in the 13-14 age group and above or to be eligible to set an American Record, per the USA Swimming Rules & Regulations, in a competition category which is different than the gender assigned to the athlete at birth," the policy states.

Nancy Hogshead-Makar, the founder of Champion Women who won three gold medals and one silver medal at the 1984 Summer Olympics, praised the new guidelines in a statement to Fox News Digital. 

"USA Swimming’s eligibility policy is outstanding for biological women," Hogshead-Makar told Fox News. "First, it declares that the purpose of the evaluation is to ensure that transgender women do not have an unfair advantage over cisgender women in competitive sports. Second, it requires that transgender women must demonstrate they’ve mitigated their male-puberty advantage."

"For long-distance events requiring large VO2 max, this requirement will not be difficult," Hogshead-Makar explained. "Distance runners and swimmers find that they lose a lot of performance fairly quickly. But for all the events in the pool, all the jumps and throws, moves requiring explosive power, it may prove to be impossible. Lia herself is showing that it is impossible; she has only mitigated her 200 yard freestyle time by 2.6%, rather than the 11.87% gap you’d expect between men and women."

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The new policy was published in USA Swimming’s operating policy manual on its website, and the organization later issued a press release on the issue. The new guidelines come as the transgender swimmer’s prominence in the pool has come under fire and a debate rages over how fair it is for her to be swimming against other biological females.

The University of Pennsylvania did not immediately respond to Fox News' request for comment.

Fox News' Andrew Murray contributed to this report.