Congratulations are in order for Chelsea Clinton and her husband, Marc Mezvinsky, because the couple is expecting their second child together! On Monday, Chelsea took to Twitter to announce the exciting news, writing, "Next summer, Charlotte is going to be a big sister! Feeling very blessed & grateful this holiday season," along with a photo of her 15-month-old daughter flipping through a book. The former first daughter and investment banker tied the knot in a gorgeous ceremony back in 2010. Congrats to the pair on their growing family!