"'Charlotte is going to be a big sister! Feeling very blessed & grateful this holiday season," wrote Clinton on Twitter Monday.
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
Clinton posted pictures on Facebook and Twitter of her 1-year-old daughter Charlotte holding a book called "Big Sisters are the Best."
"We are feeling particularly blessed ad grateful this holiday season," she wrote on Facebook. "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."
Chelsea Clinton, 35, and her father, former President Bill Clinton, have largely stayed off the campaign trail since Hillary Clinton launched her Democratic primary bid in April. Both Hillary and Bill Clinton offered congratulatory messages on social media.
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
Chelsea and Bill Clinton have said they plan to intensify their public political roles after the New Year in an effort to boost Clinton's campaign just six weeks before the first round of primary voting.
In a September interview on NBC's "Today," Chelsea Clinton said her mom was always game for singing "The Wheels on the Bus" to granddaughter Charlotte and was helping her learn to walk.
The younger Clinton is author of the book "It's Your World: Get Informed, Get Inspired & Get Going!" - an illustrated guide for young people looking to become involved in political and social causes.
Chelsea Clinton is married to hedge fund manager Marc Mezvinsky, the son of former U.S. congressman Edward Mezvinsky and former U.S. congresswoman Marjorie Margolies-Mezvinsky.
(The Associated Press contributed to this report.)