The Late Late Show host, 48, and his art dealer wife, who tied the knot in December 2008, are expecting their first child together this winter.
"Holy crackers! Mrs F is pregnant. How did that happen?" he Tweeted Wednesday. "Oh yeah I know how. Another Ferguson arrives in 2011. The world trembles."
The two are already parents to Milo Hamish, 9, Ferguson's son from a previous marriage.
Reporting by ALEXIS CHIU