Or HaChaim on Exodus 21:22/אור החיים על שמות כ״א:כ״ב

1 וכי ינצו אנשים, And if men strive together, etc. In this case each one is presumed to have murderous intent. It happpened that instead of killing his adversary the potential killer struck the woman (either fatally or otherwise).

2ונגפו אשה הרה, and they hurt a pregnant woman; the reason that the Torah uses the plural, i.e. "they hurt," is to inform us that if they both fell upon the woman thus causing her to lose the fetus they divide the compensation the woman has to be paid between them (compare Maimonides Hilchot Chovel Umazzik).

3אשה הרה, a pregnant woman, etc., the scenario is one where the woman had been present and both parties were aware of this. If the woman's presence was unknown to the parties involved in the fight, they are not guilty of any compensation. This is based on the Talmud Yerushalmi and stated explicitly in Maimonides chapter 1 of Hilchot Chovel Umazzik. Here is his wording: "If one of the parties was asleep and the other party lies down beside him, the sleeping party is free of any guilt [if his motions cause the death of the second person, Ed.] seeing that whatever harm he caused was totally accidental."

4כאשר ישית עליו בעל האשה, in accordance with what the woman's husband will impose upon him. The reason for this ruling is that the husband has the prerogative to receive compensation for the monetary value of his wife's children (such as when he sells his daughter). As a result, he is considered the litigant against the guilty party and not the mother of the unborn child.