Pregnancy rumours have followed the 26-year-old since June, and despite her best efforts arriving for the glitzy LA bash in a figure-hugging strapless dress, she could not hide her baby bump.
She followed up this arrival with husband Jordan Bratman by performing Stepping Out With My Baby in a suggestive duet with legendary crooner Tony Bennett.
Awards host Debra Messing joked that Christina had been planning to sing the same song with William Shatner but did not want to carry his "lovechild".
"Pregnancy clearly agrees with Christina," a source told the Sun.
"She looked radiant. I wish she would share her happy news with all of her fans."
Among the big winners at the Emmys was HBO's The Sopranos, which was given a swansong best drama award for its final season.
Ugly Betty's America Ferrera scooped the best actress award meanwhile, with UK comic Ricky Gervais walking away with the gong for best actor in a comedy series for BBC2's Extras.
