Like other combined birth control pills, Microgynon works by changing the hormone levels in your body. Hormonal birth control can stop ovulation, which is when the ovary releases an egg every month. If there’s no egg, there’s no pregnancy. However, ovulation doesn’t stop for everyone who takes the pill, so there are other things Microgynon does as well.
Normally, the lining of your uterus will grow every month so that it’s easier for a fertilised egg to implant itself there. On hormonal birth control, the lining stays thin. This is also why you’re likely to experience a shorter and lighter period on Microgynon. The hormones change the consistency of your cervical mucus too. The thicker mucus acts as a shield against sperm. All of these things together make it very, very unlikely that you will become pregnant while taking Microgynon.