What is the ruling regarding a pregnant woman who is unable to fast due to fear for her fetus, and does she have to make up those days after childbirth, and when should the expiation (kaffarah) be paid if it is due?

It is permissible for a pregnant or breastfeeding woman to postpone making up the fast of Ramadan until she is able, as long as she fears for herself or for her fetus/nursing infant. She is not required to pay expiation as long as she has a valid excuse.