American Airlines is offering customers a non-binary gender option when booking flights, becoming the nation’s second airline to offer the service.

In March, United Airlines was the first American air carrier to offer travelers the option of picking “U” or “X” when identifying their gender.

The airline recently completed a system update to carry four options for gender: M, F, U or X, an airline spokesperson told ABC News.

United began offering non-binary gender booking options one month after industry trade group Airlines for America and the International Air Transport Association approved a new standard of best practices that account for non-binary identification.

The federal government began requiring passengers to enter their gender and date of birth when booking flights in 2009, a security procedure that resulted from 9/11.

Gender during booking should match the ID that will be used at the TSA checkpoint, according to American Airlines. Passengers can update their gender through the airline's reservation team, and the options will be rolled out online in early 2020.

Southwest Airlines and Atlanta-based Delta Air Lines are expected to expand their gender options, according to USA Today.

More than 7,000 gender X driver's licenses and IDs were issued across nine states and the District of Columbia, USA Today reported in August.

Georgia is not one of the states offering non-binary gender designation on its driver’s licenses.