Maliks Muwatta Book 18, Hadith Number 52.

Section : The Fidya (Compensation) of a Man who Breaks the Fast in Ramadan from Weakness.

Yahya related to me from Malik that he had heard that Abdullah ibn Umar was asked about what a pregnant woman should do if the fast became 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 in place of every day, using the mudd of the Prophet, may Allah bless him and grant him peace.”

Malik said, “The people of knowledge consider that she has to make up for each day of the fast that she misses as Allah, the Exalted and Glorified, says, ‘And whoever of you is 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 a sickness.”