A binational lesbian couple has filed a lawsuit against the U.S. State Department for failing to recognize their son as an American citizen.

Allison Blixt, an American, and Stefania Zaccari, an Italian, each became pregnant in 2014, using their own eggs and sperm from an unnamed donor. Both gave birth in London in January 2015. When they went to the U.S. Embassy shortly after the boys' births, Blixt's biological son, Massimiliano, was made a U.S. citizen without any fuss.

But Zaccari's biological son, Lucas, was denied citizenship.

According to the State Department website, children born abroad are only eligible for citizenship if one of their married parents is an American citizen at the time of birth. Blixt and Zaccari were married according to British law when Lucas was born, but marriage equality didn't come to the U.S. till later that year. The U.S. Consulate therefore considers the children born out of wedlock.

After the Supreme Court struck down DOMA in 2013, gay and lesbian Americans were allowed to bring their foreign spouses to the U.S.

But the ruling did not include children of those couples, which LGBT advocates maintain is discriminatory.

"The State Department's policy, however, renders Lucas the only member of his family without the freedom to live in the U.S. permanently," the suit contends. "The State Department's decision to withhold from Lucas the same rights granted to his brother means that he will experience the indignity and stigma of unequal treatment imposed and endorsed by the U.S. government."

The women also claim they "were asked a series of invasive and legally irrelevant questions about how their children were conceived and born."

Lucas' lack of citizenship means he can't freely visit family or live in the U.S. permanently. Blixt could sponsor Lucas as his "stepmother" and bring him to the U.S. as an immigrant, but she declined. "The principle of that is just a bit too hard to swallow," she told the Washington Post . "I’m not his stepmother. I’m his mother."

Blixt and Zaccari are listed as parents on both Massimiliano and Lucas' birth certificates.

As of now the family continues to live in London. They aren't the only same-sex pair struggling with citizenship, though: American-Israeli couple Andrew and Elad Dvash-Banks are also battling the U.S. consulate for their son's right to be an American.