A woman who is pregnant or breastfeeding must make up the fasts she is unable to observe at a later time. Since she is primarily obligated to perform qaḍāʾ (make-up fasting), she is not additionally required to give fidyah.
Those who are obligated to give fidyah are individuals who will never—meaning throughout their lifetime—be able to fast. These are the very elderly and those whose illness leaves no realistic hope of recovery.