"And they hit a pregnant woman, and her fetuses miscarry" — Abba Chanin asked in the name of Rabbi Eliezer: why does the verse bother saying "a pregnant woman"? If her fetuses miscarry, it is obvious she was pregnant. The word seems redundant.
The answer creates an important legal limitation. One might think the striker is liable whenever a pregnant woman miscarries after being struck — regardless of where the blow landed. Perhaps a blow to the head or the leg that indirectly causes a miscarriage would trigger liability.
The word "pregnant" prevents this reading. By specifying "pregnant" — emphasizing her physical state — the Torah teaches that the striker is not liable unless he strikes the region of her pregnancy. The blow must land on the abdomen, on the area where the fetus is carried. A blow elsewhere on the body that coincidentally triggers a miscarriage does not generate liability for the loss of the fetus.
This seemingly redundant word thus creates a precise legal standard. Liability for causing a miscarriage requires a direct causal connection between the location of the blow and the loss of the pregnancy. A woman struck on the arm whose miscarriage may have been caused by shock, stress, or secondary effects — this falls outside the verse's scope. The Torah demands a tighter connection between cause and effect, and the word "pregnant" is the textual anchor for that requirement.