Reese Witherspoon filming "Devil's Knot" in Atlanta, Georgia on June 25, 2012. Credit: FameFlynet Pictures

She may be pregnant, but that doesn't mean Reese Witherspoon is taking things easy!

The actress, who's currently expecting her first child with husband Jim Toth, 41, began shooting Devil's Knot in Atlanta June 25. Witherspoon proudly showed off her baby bump in a mid-length purple wrap dress.

The film, which also stars Oscar winner Colin Firth, 51, "is about a crime that occurred in 1993," Witherspoon, 36, told The Chicago Sun Times Saturday. "It happened in Arkansas, and it's what I'd say is a very American story."

Based on the 2002 best-seller by Mara Leveritt, Devil's Knot tells the story of three teenagers falsely convicted of killing three 8-year-old boys during a satanic ritual. Later dubbed as the West Memphis 3, the defendants were finally freed in August 2011 after spending 18 years in jail.

Her latest project aside, Witherspoon revealed she's "very excited" about becoming a mother for the third time. (She shares custody of daughter Ava, 12, and son Deacon, 8, with ex-husband Ryan Phillippe, 37.)

"After I finish [Devil's Knot], I'm going to take a little time off," she told The Chicago Sun Times, "and go have a baby!"