Is a mother's causing the death of her fetus in the sixth month, as a result of her negligence regarding some precautions, considered accidental killing that necessitates expiation?

The basic principle is that directly causing the death of a fetus necessitates compensation (blood money) and expiation. However, as long as you are unsure whether you caused the death of the fetus, nothing is incumbent upon you. The original principle is the absolution of one's liability, and what you mentioned is not typically a cause for fetal death. Therefore, you should disregard these obsessive thoughts.