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Chelsea Clinton, Charlotte, Twitter

Chelsea Clinton Twitter

Chelsea Clinton is expecting her second child this summer, but as far as her 15-month-old daughter Charlotte is considered, there's still only one baby around!

The expecting mama chatted with an audience in Boston on Monday night at the American Library Association conference and explained that her little girl doesn't quite grasp the concept of "big sister" just yet.

"I don't think she has any idea what that means yet, but we're talking about it a lot," Chelsea explained. "She says ‘Baby,' and then points to herself!"

Chelsea Clinton, Marc Mezvinsky, Hillary Clinton, Bill Clinton

Ron Asadorian / Splash News

One way she's helping Charlotte learn a little more about welcoming a sibling is through reading—something Chelsea has held dear to her heart since she could first pick up a book.

"I read everywhere," Chelsea admitted. "Now my favorite place to read a book is curled up with my daughter on my lap, legs crossed, Indian-style."

As for Charlotte's grandparents, Bill and Hillary Clinton, Chelsea couldn't choose the better reader between the two. "They're both great readers," she told the crowd. "I'm really grateful that I don't have to [choose]!"

Meanwhile, Chelsea and her husband, Marc Mezvinsky, announced that they're expecting their second child in December. The 35-year-old took to Twitter to share a photo of Charlotte and write: "Next summer, Charlotte is going to be a big sister! Feeling very blessed & grateful this holiday season."

In response to the announcement, presidential hopeful Hillary couldn't help but gush, "Your dad and I could not be happier for you, Marc, and Charlotte. We're so excited to meet our second grandchild!"

The little one is due sometime this summer.