The N.C.A.A. on Friday cleared the way for the swimmer Lia Thomas, a transgender woman who has turned in the top times in the country in two events, to compete in the N.C.A.A. championships next month, saying that implementing new rules for testosterone testing in the middle of the season would be unfair to transgender athletes.

The move is an about-face for the N.C.A.A., the organizing body in college sports, which said last month that it would require testosterone testing for the first time and planned to adopt standards set by sports federations beginning with the winter championships, which are held in February and March.

But that desire to immediately align with federation standards became untenable last week when U.S.A. Swimming announced stringent policies that in some ways seemed targeted toward Thomas, a senior swimmer at the University of Pennsylvania.

The new policies call for halving the permissible limit for testosterone for transgender women, an extensive review of other physical characteristics, and a requirement that transgender athletes meet the new testosterone threshold — 5 nanomoles per liter — for 36 months.

The N.C.A.A. since 2011 has required transgender women athletes to be on hormone therapy for 12 months before they were allowed to compete, but had not said they would require testing until last month.

Most sports require testosterone levels in transgender women athletes to be below a prescribed level for 12 months, and a handful for as long as 24 months, though recently more experts have questioned the link between testosterone and performance.

Joanna Harper, a visiting fellow for transgender athletic performance at Loughborough University in England who is generally in favor of testosterone testing, said the 36-month requirement was extraordinary and not based on any current science, leaving Harper and others to suggest the rule was targeting Thomas, who has said she began hormone therapy more recently than 36 months ago.

A subcommittee of the N.C.A.A.’s Committee on Competitive Safeguards and Medical Aspects of Sports met on Monday to review the U.S.A. Swimming policy and forwarded its recommendation to the Board of Governors, which concurred that it would be unfair to make changes in the middle of the season.

Thomas is expected to compete next week in the Ivy League championships in Boston, where she will be the favorite in the 200 and 500 freestyle races in which she has posted the best marks in the nation this season. Thomas finished second in three men’s events as a sophomore in 2019, before she transitioned to a woman.

“As we have consistently stated, these types of midseason policy changes are unprecedented and unfair, and have the potential to put student-athletes in the impossible position of not knowing what is required in order to be eligible,” said Robin Harris, the Ivy League executive director.