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August 31, 2018 01:15 PM

Hilary Duff‘s baby-name game was over a long time ago.

The pregnant Younger actress — who has partnered with Jif and the Boys & Girls Clubs of America on a contest that’ll give a child $15,000 to fund an idea that helps others — opened up for an exclusive chat with PEOPLE about her baby girl on the way, and how she and boyfriend Matthew Koma have already chosen her moniker.

“It was the very first thing [we decided on],” said the 30-year-old star. “We’ve known her name for a really long time. Her name starts with a B, and I keep calling her baby B, which is awesome because people are like, ‘Oh, no, it’s just because of his last name.’ ” (Koma’s birth name is Matthew Bair.)

“We actually agreed,” Duff shares. “We had one name picked out and then we swapped a week later and we haven’t discussed it since.”

“It’s unique but it’s not too weird,” she continues. “The name that we first thought we were gonna use is going to be her middle name, so we’re like, ‘Let’s not stress over this anymore.’ “

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Hilary Duff
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Hilary Duff and son Luca
Hilary Duff/Instagram

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One family member who wasn’t “super excited at the beginning” of her pregnancy but has since come around a bit more to the idea of having a little sister? The star’s 6-year-old son Luca Cruz.

“He’s sweet now,” Duff says of Luca. “When we say goodbye, he always says goodbye to her. And he wants to feel her kicking. It’s really sweet. I think he’s a little nervous about sharing his toys and stuff like that, but he doesn’t realize that they’re not going to be the same thing.”

“My sister just had a baby and [Luca] was picking up her foot by the toe and he was like, ‘This is so adorable,’ ” she says of Haylie Duff‘s second daughter Lulu Gray, who arrived June 5. “He was holding her toe and lifting it up and her leg was just kind of dangling there. And he was like, ‘This is so cute and adorable. It’s so tiny.’ So at least he’s not like, ‘Ew, gross, I don’t want to touch a baby.’ “

Hilary Duff
Hilary Duff/Instagram

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Duff was convinced she was having a second son, she tells PEOPLE, since she has felt “great” throughout this pregnancy just like she did with Luca.

“I thought I was having another boy because I wasn’t sick at all,” she explains. “I feel really lucky. The first trimester was a little hard because I was shooting Younger and you’re so tired.”

“But other than that, it’s gone by very fast and I have Luca to focus on,” adds the actress. “I’m busy. So it’s been cool.”

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