It is not necessary to fast to check for delayed menstruation. The more common ways to check for delayed menstruation clinically can be through blood tests to determine if there is a pregnancy or through ultrasound to determine the thickness of the endometrium, the shape of the ovaries, and the development of the uterus and ovaries. It is not necessary to perform fasting to do lab tests and ultrasounds. There are many reasons for delayed menstruation, the most common being delayed menstruation after pregnancy, and fasting is not required to check for pregnancy. At the same time, delayed menstruation can also be caused by ovarian dysfunction, stress, insomnia, or changes in the environment, etc. Clinical blood tests can be used to determine the function of the ovaries, and fasting is not required.