Is the questioner considered sinful for fasting without her husband's knowledge and violating his oath, and does she expose herself and her fetus to peril, especially after suffering from exhaustion, dehydration, and low blood pressure during her previous fast?

The matter of a pregnant woman fasting should be referred to trustworthy doctors. If they rule that she is permitted to fast and that no harm will come to her, then she is obligated to fast, and the husband's oath holds no weight, and he must pay expiation for his oath. The wife does not sin by disobeying him because it is an obligatory fast. However, if trustworthy doctors determine that fasting will harm her or her fetus, then she has a concession to break her fast. If she trusts the word of her husband, who is a doctor, more than others, she may act upon his advice, and he is obligated to pay expiation for violating his oath.