Fertility Awareness Method (FAM) works by tracking when your body will ovulate (or release an egg) and avoiding sex on the days in which you may be at risk for pregnancy. In order to use this method successfully, it’s important to know a lot about human reproduction and your own body. In particular, eggs can live for up to 24 hours after being released and sperm can live inside the uterus for up to 6 days. This means there is a little bit of math involved in using this method to ensure you are not sexually active on any days in which sperm could survive while waiting for an egg. Furthermore, the day in which a person ovulates (or releases an egg) can change every month, so tracking more than just your menstrual cycle can be a helpful way to ensure this method is as effective as possible. Tracking your basal body temperature (or your resting temperature) and/ or your cervical mucus can also help you get a sense of when your body might ovulate.
FAM can be around 76% effective with typical use (depending on what is tracked and how consistent you are with tracking) and is free (unless you count the cost of a thermometer).