As highlighted by @Amatabhani below are the other relevant passages on the topic:
At one time there was a man had two wives, one who was barren and one who was fertile. The barren one said to a monk who was supported by her family, “If the other wife gives birth to a child, Venerable, she’ll become mistress of the whole household. Please find a method of abortion for her.” “All right,” he said, and he did so.
The child died, but the mother did not die. The monk became remorseful …
“You have committed an offense entailing expulsion.”
At one time there was a man had two wives, one who was barren and one who was fertile. The barren one said to a monk who was supported by her family, “If the other wife gives birth to a child, Venerable, she’ll become mistress of the whole household. Please find a method of abortion for her.” “All right,” he said, and he did so.
The mother died, but the child did not die. The monk became remorseful … “
There’s no offense entailing expulsion, but there’s a serious offense.”
At one time there was a man had two wives, one who was barren and one who was fertile. The barren one said to a monk who was supported by her family, “If the other wife gives birth to a child, Venerable, she’ll become mistress of the whole household. Please find a method of abortion for her.” “All right,” he said, and he did so. Both died. …
Neither died. The monk became remorseful … “
There’s no offense entailing expulsion, but there’s a serious offense.”
https://suttacentral.net/pli-tv-bu-vb-pj3/en/brahmali#127
From reading it I understand that in this kind of situation killing a child yet to be born is worse than killing its mother.
Mind that elsewhere in the Bhikkhu Vinaya we find:
“When a monk is ordained he should not intentionally deprive a living thing of life, even if it is only an ant. Whatever monk deprives a human being of life even down to causing abortion, he becomes not a (true) recluse, not a son of the Sakyans.
As a flat stone, broken in half, becomes (something) not to be put together again, even so a monk, having intentionally deprived a human being of life, becomes not a (true) recluse, not a son of the Sakyans.
This is a thing not to be done by you as long as life lasts."
https://suttacentral.net/pli-tv-kd1/en/horner-brahmali