Clinton during the Education Nation Summit in 2013. Credit: Rob Kim/NBC/NBCU Photo Bank via Getty Images

Chelsea Clinton’s daughter, Charlotte, is in for the shock of her life. Come this summer, the 18-month-old will no longer be the only child.

“I think she has no idea,” Chelsea told Access Hollywood on Sunday, April 3, at the Clinton Foundation Day of Action event in Oakland, California. “We talk to her a lot about the fact that she’s going to become a big sister, and thankfully there are these wonderful books that are clearly written to explain what it means to become a big sister.”

But there’s one problem: “She really doesn’t understand, because every time we get to the part where the new baby comes, [Charlotte] goes, ‘Baby!’ and points to herself.”

The former first daughter and her husband, Marc Mezvinsky, announced on December 21, 2015, that they are expecting their second baby. 

Though the 36-year-old is in her second trimester, she’s been on the campaign trail with her mother, Hillary Clinton, who is the front-runner for the Democratic Presidential Nomination. Hillary — whose whose husband, Bill Clinton, served as president from 1993 to 2001 — has said if she moves back into the White House, she’ll set up a playroom for her grandkids.

“It’s been such a joy for me to watch my parents as grandparents,” Chelsea gushed to Access Hollywood. “I didn’t know that was a gift that Charlotte would give me. Watching them be so loving and invested in every part of her life and wanting to know every day what she’s up to and how she’s doing … It is so heartwarming.” 

The It’s Your World: Get Informed, Get Inspired & Get Going! author and Mezvinsky welcomed Charlotte in September 2014. "My whole life is reoriented around my daughter in the most blessed sense,” she said in the May 2015 issue of Elle. "I now understand — this is something else that Marc and I talk about all the time — all of the enthusiastic, bombastically spectacular, wonderful things people say about their children, because we also feel and think all those things about Charlotte."

She joked: "Marc and I are like, 'What did we do before we were parents?'"