The two were guests on The Ellen DeGeneres Show on Monday, where the father of five revealed Hilaria "wants one more" baby.
"She wants to have a girl," the 60-year-old actor said. "She wants Carmen to have a little sister."
Alec shares four children -- daughter Carmen, 5, and sons Rafael, 3, Leonardo, 2, and Romeo, 8 months -- with Hilaria. He also shares 23-year-old daughter Ireland with his ex-wife, Kim Basinger.
Later on the show, Hilaria confirmed that she would like another baby, and really wants a girl.
"We gotta keep on trying, Ellen," Hilaria, 35, said when asked what would happen if they don't have a girl. "I'm serious ... when I told Carmen -- everything revolves around her -- that I was pregnant with another boy she said, 'Mommy, how about you have another girl, then you can have another boy.' And I said, 'It doesn't work that way.'"
"She asks me all the time when she's going to have her sister," she added.
For now, Alec and Hilaria are spoiling all their little ones. Back in October, the lovebirds brought Romeo to the red carpet premiere of The Kindergarten Teacher in East Hampton, New York, when he was just four months old.
The outing came just a few days before they brought the rest of the kiddos they share together to the Chairman's Reception for the Hamptons International Film Festival.