One of the nation's most prominent legal nonprofits declared that a person could be a man even if they get pregnant or menstruate.

"There's no one way to be a man," the American Civil Liberties Union (ACLU) tweeted on Tuesday. The tweet came on International Men's Day and amid a wave of legal challenges surrounding gender.

"Men who get their periods are men. Men who get pregnant and give birth are men," the group added. The ACLU's message garnered around 19,000 comments and was transmitted to its 1.7 million followers.

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The ACLU has been at the forefront of challenging President Trump's agenda, as well as pushing controversial legal provisions for people who identify as transgender.

The controversial organization has also defended Medicaid funding for gender surgeries and allowing individuals to change the gender marker on their birth certificates. The group also sued a Catholic health organization over its refusal to cover gender-related surgeries, leading to a settlement in which the organization agreed to change its medical plan.

In 2015, the group intervened in a case surrounding gender and locker rooms. A blog post from the ACLU of Illinois claimed that "a transgender girl is female. She is a girl through and through -- not something in between."

The post took on an Illinois superintendent's claim that their client had a "male body" and suggested the school district's position ran counter to science.

“The superintendent asserts that a girl who is transgender has a ‘male body’ and that transgender students are ‘of the opposite sex’ when defending the District’s position," the post read.

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“Such assertions are wrong as a matter of science and offensive because they serve to challenge and undermine the very core of a person’s identity."

It's unclear what criteria the ACLU uses to determine who is and who is not a man. The group did not respond to Fox News' request for comment.

According to its annual report, it received $285,790,662 in total support and revenue for FY2018. Its foundation, a 501(c)(3) charitable organization, received $145,688,713.

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The organization's advocacy has provided a stark contrast to the Trump administration's reported view of gender. At the end of last year, the group condemned the administration over a reported rule that would interpret sex discrimination according to biological factors, rather than gender identity.

It also joined a lawsuit challenging the administration's rule granting conscience protections for health care professionals who refused to perform certain procedures.