Women are only fertile for a limited amount of time during each cycle. Sympto-thermal fertility tracking methods, as described in the book "Taking Charge of Your Fertility" by Toni Weschler, empower women to understand exactly when in their cycles they are fertile.
Understanding your personal cycle of fertility means you can accurately predict your period, understand precisely when you have ovulated, know when you are fertile and infertile in your cycle, and avoid the side effects of hormonal birth control.
I have personally used sympto-thermal fertility tracking as my exclusive method of birth control within a relationship for 2 years.