Hazrat Umar bin Abdul Aziz (may Allah have mercy on him) asked the scholars and callers of Madinah, and they said: "The waiting period (istibra) of a pregnant woman is not less than three months." And Umar bin Abdul Aziz (may Allah have mercy on him) liked this statement.
Hadith Referenceسنن سعید بن منصور / كتاب الطلاق / 3379