What is the ruling regarding a day a woman broke her fast, believing she had to do so in order to undergo necessary medical tests for pregnancy, and is expiation required of her?

She must make up the day she missed. It is hoped that she will not bear sin if she thought breaking the fast was obligatory for the purpose of analysis [of her husband's triple divorce] and it was not possible to do it at night. If this was not the case, then she must make up the day and repent to Allah the Almighty. There is no expiation upon her as long as the breaking of the fast was not due to sexual intercourse.