"Marc and I have loved watching Charlotte be such a wonderful big sister and we're excited to watch Aidan become a big brother! We cannot wait to meet our newest addition later this summer," she tweeted.
She did not reveal an exact due date for the child.
The daughter of former President Bill Clinton and former Secretary of State Hillary Clinton is an author, television personality and board member for the Clinton Foundation. She married investment banker Marc Mezvinsky in 2010. Their daughter Charlotte was born in 2014 and their son Aiden in 2016.