Ellen, who incorrectly guessed the gender of Natalie's first child when she was pregnant, said that she felt it was a girl this time round, and that the baby should be called Ellen, regardless of the sex.
"It's a good name, Aleph and Ellen," said the presenter, referring to Natalie's five-year-old son.
"I have no idea," the actress replied. "I really don't know the gender!"
Natalie Portman is expecting her second baby with Benjamin Millepied
Natalie opened up about her second pregnancy, revealing that her cravings are different to when she was expecting her son.
"With him I wanted like cream puffs or any dessert with cream in it, or just plain whipped cream straight," she said. "And this time it's like healthy pineapple, cucumber, burritos – which is maybe less healthy – but delicious!"
When asked about her son's unusual moniker, Natalie, 35, explained: "It's the first letter of the Hebrew alphabet and it's not really a name in Hebrew usually, but it has a lot of mystical symbolic meanings of humility, strength."
Ellen made a $1,000 punt that Natalie is expecting a baby girl
The Israel-born star and her husband Benjamin Millepied are set to become parents-of-two early next year. The couple met in 2009 while filming Black Swan, for which Natalie won an Oscar.
The Harvard University alum is making a return to screens soon, playing the iconic Jackie Kennedy in biopic Jackie. She spent a month working with a dialect coach, perfecting Jackie's distinct voice.
"I think I knew her like most Americans know her," said Natalie of the former First Lady. "The icon, what she looked like, the fashion, the style, the broad strokes of her life... but this was my first time approaching her as a human being."