Chelsea Clinton is expecting her second child with husband Marc Mezvinsky. The pair are already parents to 15-month-old daughter, Charlotte.

The former First Daughter announced the news on Monday, by posting a black-and-white photo of Charlotte holding a big sister book on Twitter.

Next summer, Charlotte is going to be a big sister! Feeling very blessed & grateful this holiday season. pic.twitter.com/gpCGqcmeCq

- Chelsea Clinton (@ChelseaClinton) December 21, 2015

Later on, the mom-to-be expanded on her announcement with a Facebook post from both her and Mezvinsky.

"Next summer, Charlotte is going to be a big sister! We are feeling particularly blessed and grateful this holiday season. Being Charlotte's parents has been the most joyous, meaningful and loving experience of our lives and we are very excited about growing our family in the new year."

Following her announcement, both Hillary and Bill Clinton took to their respective Twitter accounts to congratulate their daughter on her growing family.

Your dad and I could not be happier for you, Marc, and Charlotte. We're so excited to meet our second grandchild! -H https://t.co/hgTr07HBlr - Hillary Clinton (@HillaryClinton) December 21, 2015

Christmas comes early! @HillaryClinton and I are thrilled for Chelsea, Marc and Charlotte's growing family in 2016! https://t.co/D1zCrrzXa2

- Bill Clinton (@billclinton) December 21, 2015

Chelsea's pregnancy announcement comes just a few months after she shared her desire to have another child in an interview with People magazine.

"I think my love for Charlotte has just opened more space in my heart and my brain that I didn't even know was there," she said.

Earlier today, Chelsea posted a candid photo of herself with Charlotte, expressing her gratitude for all the well-wishes she received from her pregnancy announcement.

Chelsea, 35, and Mezvinsky, an investment banker, have been married since July 2010.