When a pregnant woman is murdered and the fetus dies, it's not always considered a double homicide. The charge could be 1st-degree murder for the death of the woman, and 2nd-degree murder or manslaughter for the death of the fetus. The charge will be based on the motive, intent, and malice of the killer.
In most cases, the killer planned and INTENDED to kill the woman. That's 1st-degree premeditated murder in her death.
The killer didn't specifically INTEND or WANT to kill the fetus - it died as a result of the woman's death. That's 2nd-degree murder or manslaughter in the death of the fetus.
When Scott Peterson killed his wife and unborn child, he was convicted of 1st-degree capital murder for her death, and given the death penalty. He was convicted of 2nd-degree murder for the death of their unborn child. He INTENDED and PLANNED to kill his wife; he wanted her dead at all costs. He didn't INTENTIONALLY plan or want to kill their unborn child (even though he knew that would be the result).
1st-degree capital murder can carry the death penalty. 2nd-degree murder or manslaughter does not.