Clinton made the announcement at a speech in New York City her mother was giving.
Shortly after the announcement, Hillary Clinton tweeted, "My most exciting title yet: Grandmother-To-Be! @billclinton and I are thrilled that Chelsea and Marc are expecting their first child!"
Chelsea Clinton is married to investment banker Marc Mezvinsky. Clinton said at the speech they do not yet know whether the baby is a boy or a girl.
Bill Clinton tweeted this afternoon, "Excited to add a new line to my Twitter bio...grandfather-to-be! @hillaryclinton and I are so happy for Chelsea and Marc!"
For more, read this full ABC News report.
Click here to follow Daily Voice Greenburgh and receive free news updates.