Until the order, which Trump signed the same day he was inaugurated as the 47th president, the U.S. government has, at least the late 1800s, considered the child of any immigrant born on U.S. soil an automatic citizen, even to a mother in the United States illegally.

Eighteen states, plus the District of Columbia and San Francisco sued in federal court to block Trump's order.

The executive order restricts automatic citizenship for babies born to parents who aren't citizens or lawful, permanent residents.