As a result of the different hormones that are released during the menstrual cycle, there are several indicators that can determine a woman's fertility status and where she is in her cycle. Natural Cycles uses basal body temperature (BBT) and LH measurements as these indicators are the most scientifically accurate and objective measurements as well as being the easiest to track.
Basal body temperature (BBT)
BBT is your resting body temperature. A rise in BBT (about 0.3°C or 0.5°F) coincides with an increase in the hormone progesterone. This occurs once a woman has ovulated and has entered the luteal phase - the infertile stage of the cycle.
Luteinizing Hormone (LH) tests (optional), also known as ovulation tests, measure the amount of LH in a woman’s urine. LH levels rise roughly 48h prior to ovulation and initiate the ovulation. A positive result on an LH test, followed by elevated temperatures, confirms that ovulation has occurred.
Quantitative indicators reduce human error
Natural Cycles is a simple and user-friendly method of contraception. This means we have decided to solely include BBT and LH as data entry points. The app's sophisticated algorithm takes a woman's individual menstrual cycle patterns and monthly variations into consideration including a woman's cycle history, cycle irregularity, temperature fluctuations, recent usage of hormonal contraception, ovulation day variation, sperm survival.
Other well-known indicators, like fern tests or observing cervical fluids (also known as mucus or discharge), requires a lot of practice and often lead to user error where analysis of each of these indicators is highly subjective. Using BBT and LH provides users with the most accurate results whilst being the most simple and user-friendly way of tracking your fertility. If you would, however, still like to track other fertility signs then you can add them to the "Notes" section of your daily data entry.