Sayyiduna Abu Sa'id (may Allah be pleased with him) says that the Messenger of Allah (peace and blessings be upon him) was asked about a camel and a cow in whose womb there is a fetus, and he (peace and blessings be upon him) said: "When you have mentioned the name of Allah over the slaughtered animal, the slaughter of its mother counts as the slaughter of its fetus."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الضحايا / 19491