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How Does Natural Cycles Work?

The Natural Cycles algorithm analyses your body temperature to tell you where you are at in your cycle and when you need to use protection or abstain from sex to prevent a pregnancy.

You measure, we analyse

You measure your temperature first thing in the morning before you get up and out of bed (we recommend doing this at least five mornings a week). This reading is an indirect measure of the hormone levels in your body.

The Natural Cycles algorithm analyses the temperature measurements you enter into the app to detect ovulation, thereby identifying green days, when no protection is needed, or red days, when you should use condoms or abstain from sex to prevent a pregnancy.


It’s all in the digits

You will need a two decimal basal thermometer, which is not the same as a normal fever thermometer. Basal thermometers are more sensitive than regular fever thermometers as they show two decimals (e.g. 36.72°C / 98.11°F). The range of how much your temperature changes is actually quite small, so for Natural Cycles to work, you will need the correct type of thermometer.

To measure, you place it under your tongue as far back as possible, and press the button to turn it on. Once it beeps, enter your reading into the app.

36.62 degrees celsius shown on Natural Cycles thermometer
Natural Cycles Network Information/Brain

The “brain” behind the app

Natural Cycles is powered by a smart algorithm that can accurately determine your daily fertility status. It is smart because it adapts to your unique cycle pattern by learning over time as you add more data.

The Natural Cycles algorithm has been intricately designed to account for sperm survival, variation in cycle length, ovulation day and temperature fluctuations. It is sensitive to subtle patterns in your cycle: if it sees something unexpected, it will err on the side of caution and give you a red day.

Period data

For the algorithm to calculate your daily fertility, you also need to add your period data each month.

Use protection or abstain on Red Days

The Natural Cycles algorithm will analyse your temperature measurements and tell you which days you need to use protection or abstain from sex to prevent a pregnancy. The number of green days increases as the algorithm gets to know your individual cycle pattern. 

The more dedication you put into using the app, the more green days you’re likely to get back. It can take around 3 months of using Natural Cycles consistently for the algorithm to get to know your unique cycle.

Just keep in mind that Natural Cycles does not protect against Sexually Transmitted Infections (STIs) in any way.

Use protection screen

Even more Green Days

You can also add additional data to the app like LH test results. LH stands for Luteinising Hormone and LH tests (also known as ovulation tests) help detect ovulation. This is a simple urine test you only need to take on a few days of your cycle. They can help increase your number of green days, particularly in the beginning when the algorithm is just getting to know your cycle, but they are not essential for the app to work effectively. You can purchase some in our webshop.