Lia Thomas became historic in March with her victory in the college championship's 500 yards free by being the first person born to win a women's class.
Thomas will not be allowed to do that again.
The International Swimming Federation held an extra congress in Budapest and voted on what the competition rules for transgender people should look like in swimming in the future.
It was concluded that you should have completed your gender correction before the age of twelve to compete with women.
196 voted in favor, 42 against and 36 abstentions.
A working group has also been appointed to investigate so-called open classes for trans people.