My wife is pregnant and the doctor advised her not to fast. Which is permissible for her, makingkaffara [expiation] or fidyah [paying compensation]? In either case, what is its sum and when is it due?

If a Muslim physician determines that the inquirer's wife is unable to fast, then there is no objection to her refraining from fasting. She must make up her missed fasts after her excuse ends by fasting a day for each unperformed fast day.

If she is unable to fast even after her excuse ends, then she must feed one poor person two meals for each unperformed fast based on an average standard of the food she feeds her family.
The above answers the question and Allah Almighty knows best.