Al-Zuhri and al-Hasan (may Allah have mercy on them) said concerning a man who said to his slave woman: "You are free, but not what is in your womb": Both she and what is in her womb are free; his exception does not count. Ma'mar said: Someone who asked al-Hakam told me that he said the same.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب العتق / 21367