How do you not get pregnant without using contraception?

The only sure way is to not have vaginal intercourse. You can also do it on the days when you are sure you do not ovulate and also not a week before that since the sperms can live inside for a week. That means you need to see a doctor to find out exactly when you ovulate and make a schedule and test the urine often since only 12% ovulate on the same day each month.