Ibn Abbas (may Allah be pleased with him) said that Umar (may Allah be pleased with him) asked the people about the fetus. Haml bin Malik bin al-Nabighah stood up and said: "I was between two wives of mine; they fought, and one of them struck the other with a pole while she was pregnant, so the woman died and the child too. He (peace and blessings be upon him) gave judgment in it that 'blood money be given for the child, and the woman be killed for the woman.'"
The chain of this hadith is sahih, as Imam al-Tirmidhi (may Allah have mercy on him) mentioned in Kitab al-'Ilal. He said: I asked Imam al-Bukhari (may Allah have mercy on him) about this hadith, and he said: "It is sahih."
Al-Shaykh (may Allah have mercy on him) said: It is as al-Bukhari said regarding the hadith being connected through the mention of Ibn Abbas in it, except that its wording contains an addition that I did not find in any of the routes of this hadith, namely the killing of the woman for the woman. In the hadith of Ikrimah from Ibn Abbas (connected), the hadith of Ibn Tawus from his father (mursal), and the hadith of Jabir and Abu Hurayrah (connected), it is established that he ruled that her blood money was to be paid by the 'aqilah (the killer's paternal kin).
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الجنايات / 15986