A federal judge in the US city of Detroit ruled unconstitutionally on a law banning FGM, and also dismissed several charges against two doctors and others in a case involving female genital mutilation.
The District Court Judge, Bernard Friedman, said that Congress lacked the authority to take the ban, noting that the authority to ban female genital mutilation dates back to the states.
Jenna Palaya, a spokeswoman for Detroit federal prosecutor Matthew Schneider, said the prosecutor's office would review the verdict before making a decision on the appeal.
The decision overturned the main charges against Jumana Nagarwala, a doctor who circumcised 9 girls from Michigan, Illinois and Minnesota and another medical clinic in the suburb of Livonia, Detroit.
Four of the eight defendants in the case were excluded, including 3 of 4 mothers charged with having their daughters circumcised.
A 7-year-old girl told investigators that she and another girl had been taken to Detroit for what they thought was a "special trip for girls" and had been told not to discuss circumcision once they were finished.
"We are very happy with today's verdict, but the sweetness of victory is bitter because we fully expected our client to be acquitted of these charges in the trial," said Molli Blythe, lawyer for Ngarwala.
Civil liberties groups say 27 US states also prohibit the practice, while Michigan joined the list last year.
The World Health Organization (WHO) estimates that more than 200 million girls and women are currently being circumcised.
Ngarwala last month pleaded not guilty to the two remaining charges - obstructing a formal procedure and conspiring to travel in order to engage in prohibited sexual activity.
