[A woman causing her pregnancy to be miscarried]

The answer – and Allah is the One who grants success – is that they have said, as in the marginal notes on Sharḥ al-Azhār: “There is nothing (no liability) upon one who dies due to the killing of his mother if he has not yet separated (from her), and they did not consider movement (in the womb) to entail certain knowledge (of life).”
I say: It is as if they built this ruling upon the established legal principle: “The basic presumption is freedom from liability (barāʾat al-dhimmah).” They have applied this principle here, and nothing has arisen to overturn this basic presumption. The movement that occurs in the womb of a pregnant woman yields only conjecture, whereas freedom from liability is certain; and it is not permissible to abandon what is certain on the basis of what is conjectural. Similar to this is their statement: “The certainty of purity or impurity is not lifted except by certainty.”
Yes, such a woman ought to repent to Allah and seek His forgiveness.
If it is said: Then she ought also to pay the diyah and fast two months.
We say: We did not state that, for two reasons:
1 – Because the woman’s pregnancy, so long as it has not separated (from her), is like a part of her body; so if she commits an offence against it while it is in her womb, it is as if she has committed an offence against herself. One who commits an offence against himself is only required to repent and seek forgiveness.
The proof of this is that in the case of one who kills a pregnant woman whose fetus has already moved, only the diyah of the woman is obligatory, not the diyah of what is in her womb – and that is if the fetus dies in her womb because of her death.
2 – That the Messenger of Allah (May Allah bless him and his family and grant them peace) did not make for the fetus of a woman, which emerges dead due to an assault upon its mother by beating, anything except a ghurra (a specific monetary compensation), and did not oblige fasting.

Source: Min Thimār al-ʿIlm wa al-Ḥikmah vol.2