"Josh & Me & BABY makes three!!!" she Tweeted.
This will be the first child for the couple, who wed in January 2009 in Malibu.
Both Fergie, 37, and Duhamel, 40, have been clear about their desire for children – but kept the timeline open.
"Not right this second," the Black Eyed Peas frontwoman said in May 2011, while her husband added, "We'll go there at some point."