Musannaf Ibn Abi Shayba 9:1402

Ibn Idris narrated to us, he said: Ibn Abi Layla used to say: The *mukātabah* (a slave woman who has a contract of manumission) reverts to being an *umm walad* (a slave woman who has borne a child to her master). That is, if he has intercourse with her and she gives birth.