What is the ruling on a nurse who has an IUD (intrauterine device) working during her fast?

A nurse's fast is not invalidated by inserting an IUD because there is no evidence to suggest that the fast of someone who looks at another's 'awrah is invalidated. The default is that the fast remains valid. It should be noted that it is not permissible for women to have an IUD inserted unless there is a pressing necessity.