Ikrimah (may Allah have mercy on him) said that Sayyiduna Umar (may Allah be pleased with him) said: The umm walad becomes free, even if the child is stillborn.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب عتق أمهات الأولاد / 21798
Al-Hasan (may Allah have mercy on him) said: If an umm walad miscarries something known to be from a pregnancy, she is set free by it and is counted as an umm walad.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب عتق أمهات الأولاد / 21799