Rumors that the Once Upon a Time actress was pregnant began a few weeks ago as the 35-year-old has been spotted wearing loose-fitting clothing for a while.
Details like the baby's gender and the due date are being kept private.
Goodwin and Dallas, 31, met on the set of the ABC show in 2011. The couple went public with their relationship the following spring. The two announced their engagement in October 2013.
In November 2010 -- when she was engaged to actor Joey Kern -- Goodwin told InStyle UK that she wants to have a big family.
"I want three or five children, but I need a bigger house or a smaller closet. I'm 32, so I'm already thinking about that. If it happened right now, I would absolutely have a baby."
"The only issue I have with aging is nothing to do with my career," the Killing Kennedy star added. "It's that I want to have a lot of kids before I'm physically unable to do so."