FIFA regulations ensure it. The question is why?
Since 2011, teams have been required to sign a declaration guaranteeing that players nominated to the World Cup were "of an appropriate gender."
According to FIFA, "It lies with each participating member association to … ensure the correct gender of all players by actively investigating any perceived deviation in secondary sex characteristic."
How verification takes place isn't clear, although physical exams, hormone and chromosome testing have been used in the past.
Reports out of Germany say the German Football Association used players' gynecological reports as evidence.
German team manager Doris Fitschen told the Bild newspaper, "We are glad to be able to confirm that all our players are female."
Midfielder Lena Goebling stated she and her teammates "have all been very amused about the test and have not taken the whole thing so seriously."
But gender testing can be humiliating. Women who don't conform to perceived feminine norms are often scrutinized based on their physical appearance alone.
In 2013, South Korean player Park Eun-seon was accused of being a man by six rival teams in the South Korean women's league, all demanding she submit to a gender test.
And in 2008, Equatorial Guinea's Genoveva Anonma was forced to undergo a physical examination to prove her gender after scoring the winning goal in the African Women's Championship.
Are men getting tested?
Former Canadian team captain Geri Donnelly said she was "absolutely stunned" to hear of the gender verification rule.
"The good part about the women's game is that it represents women so well," Donnelly said. "What's on the pitch is what's in society, in terms of what they look like. Every fan, every little girl can relate to at least one player on a team."
"It's a bit of a discriminatory thing." said Katie Thorlakson, who played for Canada in the 2007 World Cup. "Are the men playing on [artificial] turf? No. Are the men getting tested for gender? No."
FIFA rules do call for gender verification of all players, although there is no record of male players ever being tested.