Yahya related to me from Malik that he had heard that Abdullahibn Umar was asked about what a pregnant woman should do if the fastbecame difficult for her and she feared for her child, and he said,"She should break the fast and feed a poor man one mudd of wheat inplace of every day, using the mudd of the Prophet, may Allah bless himand grant him peace."Malik said, "The people of knowledgeconsider that she has to make up for each day of the fast that shemisses as Allah, the Exalted and Glorified, says, 'And whoever of youis sick or on a journey should fast an equal number of other days, 'and they consider her pregnancy and her concern for her child as asickness."
USC-MSA web (English) reference: Book 18, Hadith 52
Arabic reference: Book 18, Hadith 685