Hormone levels in your body rise and fall over the course of your menstrual cycle. This tells your body to do certain actions at certain times of the month.
One of these is ovulation. This is where an egg is released by your ovary, ready to be fertilised.
The progestogen hormone in the pill tricks your body into thinking that it’s already ovulated. So no egg is released (and it can’t be fertilised by sperm if it’s not there).
Mini pills don’t do this all the time, so there are two additional things they do to make pregnancy less likely.
One is that they make your cervical fluid thicker. Sperm then has a harder time getting through to an egg to fertilise it.
Another is that they stop the lining of the uterine wall from building up with blood. A fertilised egg would need to implant here to develop. With no lining here, the egg can’t attach.
So as long as you take the pill right, these three functions have got you covered to a very high degree.
If you miss the odd pill though, there’s a bigger chance it may not work properly. Each missed dose reduces this level of protection significantly, with the NHS estimating that typical use averages around 91% protection.