There's a very similar question on Microgynon usage, and chances of pregnancy here.
Essentially, you are protected from getting pregnant after 7 days of consistent use of birth control pills. Consistent usage means that you’re taking the pill every day.
If taken correctly, Microgynon protects against pregnancy 99% of the time. This means that less than 1 of 100 women will get pregnant with perfect use of the pill.
Microgynon (when taken correctly) works by preventing the egg from being released, so even if your boyfriend ejaculates inside you, there is no egg present for the sperm to fertilise.
