Musannaf Ibn Abi Shaybah

Hazrat Qatadah (may Allah have mercy on him) says that if a person says to his wife, "When you become pregnant, you are divorced," then the man should not have intercourse with her again after having intercourse in each period of purity, until she becomes pure after menstruation again. When her pregnancy becomes apparent, she will become separated (divorced).

Hasan (may Allah have mercy on him) says that if a man says to his wife, "When you become pregnant, you are divorced," then after the woman becomes pure from her menstruation, he may have intercourse with her and then refrain. Ibn Sirin (may Allah have mercy on him) says that he may have intercourse with her until her pregnancy becomes apparent.