Norethisterone is a synthetic progestogen that is similar to the progesterone hormone we naturally produce. During a menstrual cycle, the level of progesterone hormone falls which causes your womb lining to shed and your period to occur. If progesterone - or norethisterone - is taken throughout, the levels don't drop which prevents menstruation from taking place.
This is a companion discussion topic for the original article at https://patient.info/news-and-features/how-to-delay-period-norethisterone