Every month, your body goes through a checklist of things to prepare for pregnancy. During ovulation, the ovary releases an egg which travels through the fallopian tubes towards the uterus. If the egg is fertilised as it travels, it attaches to the uterine wall and starts growing. If it isn’t fertilised, you get your period and it’s better luck next time.
All of this happens because of rising and falling levels of hormones. Hormones trigger ovulation as well as smaller changes, like telling the wall of the uterus to become thicker so that it’s easier for a fertilised egg to attach itself there.
The hormone in Noriday, norethisterone, is a type of progestogen. Adding it to the hormones already in your body alters what happens every month. Norethisterone increases the density of cervical fluid, so sperm cannot swim through and reach an egg as easily. Some of the time, the progestogen in mini pills totally overrides the normal hormonal cycle, preventing ovulation, so fertilisation can’t happen.