How Natural Cycles works

Natural Cycles is the only certified contraceptive app.

Based on an advanced bio-statistic algorithm, Natural Cycles analyzes a woman's basal body temperature (BBT) and Luteinizing Hormone (LH) data to identify her fertile and non-fertile days. This is by analyzing the BBT to confirm that ovulation has occurred and the woman is no longer fertile. By measuring and adding the basal temperature in the morning (5 values per week is recommended) the app gets to know the woman and her unique menstrual cycle. After 1-3 cycles of measuring the app has gotten to know her well enough to be able to confirm when ovulation has occurred.

The key is to know when a woman ovulates. 
Ovulation is the day a woman can get pregnant as it is the phase of the menstrual cycle when an egg is released from the ovaries and can be fertilized by sperm. Since sperm can survive up to 5 days in the female body a woman is only fertile for ~6 days per cycle. 

The app then gives out red or green days according to fertility status for the day.
Red days: You are fertile and should use protection during intercourse.
Green day: You are not fertile and do not need to use protection during intercourse.

Natural Cycles uses two well-known indicators of ovulation to confirm it has occurred: 
a) Basal Body Temperature (BBT) 
A rise in temperature (0.2-0.45C) is observed after ovulation, as the progesterone is released
b) Luteinizing Hormone (LH) 
A surge in LH hormones occurs two days prior to ovulation and marks the most fertile days.

Natural Cycles uses temperature as the primary indicator and recommends using LH tests to quickly detect and confirm ovulation. We see that Natural Cycles Prevent users who add positive LH tests gets roughly 5% more green days per cycle. For users who Plan a pregnancy, it helps the algorithm narrow down the most fertile days.

What about irregular cycles? 
No worries! Natural Cycles gets to know you and your unique cycle, among many other factors and will adapt to your cycle. Cyclers with irregular cycles may experience more red days, but the efficacy of the app is the same.

On this photo, you can see the temperature throughout the cycle, and the increase in temperature that occurs after ovulation. You can also see the hormone levels throughout the cycle, and how it affects the basal temperature.