Prosecutors are adding a sexual assault count against the suspect, 22-year-old Joshua Graham Packer.
A new grand jury indictment alleges Packer used a knife to force Davina Husted to give him oral sex before he stabbed her to death.
Husted was pregnant at the time. Packer is also accused of killing her husband, Brock.
Officials said DNA evidence led to the latest indictment. Packer faces the death penalty if convicted.